systemctl: allow suppress the warning of no install info using --no-warn

In cases like packaging scripts, it might be desired to use
enable/disable on units without install info. So, adding an
option '--no-warn' to suppress the warning.

man/systemctl.xml

@@ -771,6 +771,9 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
             account.
             </para>
 
+            <para>When using this operation on units without install information, a warning about it is shown.
+            <option>--no-warn</option> can be used to suppress the warning.</para>
+
             <para>Enabling units should not be confused with starting (activating) units, as done by the
             <command>start</command> command. Enabling and starting units is orthogonal: units may be enabled without
             being started and started without being enabled. Enabling simply hooks the unit into various suggested
@@ -814,8 +817,8 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
             executed. This output may be suppressed by passing <option>--quiet</option>.
             </para>
 
-            <para>This command honors <option>--system</option>, <option>--user</option>, <option>--runtime</option>
+            <para>This command honors <option>--system</option>, <option>--user</option>, <option>--runtime</option>,
-            and <option>--global</option> in a similar way as <command>enable</command>.</para>
+            <option>--global</option> and <option>--no-warn</option> in a similar way as <command>enable</command>.</para>
           </listitem>
         </varlistentry>
 
@@ -1997,6 +2000,17 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--no-warn</option></term>
+
+        <listitem>
+          <para>Don't generate the warning shown by default when using
+          <command>enable</command> or <command>disable</command> on units
+          without install information (i.e. don't have or have an empty
+          [Install] section).</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--no-block</option></term>
 

src/systemctl/systemctl-enable.c

@@ -67,7 +67,7 @@ int verb_enable(int argc, char *argv[], void *userdata) {
         InstallChange *changes = NULL;
         size_t n_changes = 0;
         int carries_install_info = -1;
-        bool ignore_carries_install_info = arg_quiet;
+        bool ignore_carries_install_info = arg_quiet || arg_no_warn;
         int r;
 
         if (!argv[1])

src/systemctl/systemctl.c

@@ -84,6 +84,7 @@ bool arg_show_types = false;
 int arg_check_inhibitors = -1;
 bool arg_dry_run = false;
 bool arg_quiet = false;
+bool arg_no_warn = false;
 bool arg_full = false;
 bool arg_recursive = false;
 bool arg_with_dependencies = false;
@@ -277,6 +278,8 @@ static int systemctl_help(void) {
                "                             kexec, suspend, hibernate, suspend-then-hibernate,\n"
                "                             hybrid-sleep, default, rescue, emergency, and exit.\n"
                "  -q --quiet             Suppress output\n"
+               "     --no-warn           Don't generate warning when trying to enable/disable\n"
+               "                         units without install information\n"
                "     --wait              For (re)start, wait until service stopped again\n"
                "                         For is-system-running, wait until startup is completed\n"
                "     --no-block          Do not wait until operation finished\n"
@@ -433,6 +436,7 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                 ARG_READ_ONLY,
                 ARG_MKDIR,
                 ARG_MARKED,
+                ARG_NO_WARN,
         };
 
         static const struct option options[] = {
@@ -465,6 +469,7 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                 { "no-wall",             no_argument,       NULL, ARG_NO_WALL             },
                 { "dry-run",             no_argument,       NULL, ARG_DRY_RUN             },
                 { "quiet",               no_argument,       NULL, 'q'                     },
+                { "no-warn",             no_argument,       NULL, ARG_NO_WARN             },
                 { "root",                required_argument, NULL, ARG_ROOT                },
                 { "image",               required_argument, NULL, ARG_IMAGE               },
                 { "force",               no_argument,       NULL, 'f'                     },
@@ -926,6 +931,10 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                         arg_marked = true;
                         break;
 
+                case ARG_NO_WARN:
+                        arg_no_warn = true;
+                        break;
+
                 case '.':
                         /* Output an error mimicking getopt, and print a hint afterwards */
                         log_error("%s: invalid option -- '.'", program_invocation_name);

src/systemctl/systemctl.h

@@ -65,6 +65,7 @@ extern bool arg_show_types;
 extern int arg_check_inhibitors;
 extern bool arg_dry_run;
 extern bool arg_quiet;
+extern bool arg_no_warn;
 extern bool arg_full;
 extern bool arg_recursive;
 extern bool arg_with_dependencies;