shared/install: ignore Alias in [Install] of units which don't allow aliases

A downside is that a warning about missing [Install] is printed:
$ systemctl --root=/ enable mnt-test.mount
[/etc/systemd/system/mnt-test.mount:5] Aliases are not allowed for mount units, ignoring.
The unit files have no installation config (WantedBy, RequiredBy, Also, Alias
settings in the [Install] section, and DefaultInstance for template units).
This means they are not meant to be enabled using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
   .wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
   a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
   D-Bus, udev, scripted systemctl call, ...).
4) In case of template units, the unit is meant to be enabled with some
   instance name specified.

That's a bit misleading, but I don't see an easy way to fix this. But
the situation is similar for many other parsing errors, so maybe that's
OK.

src/shared/install.c

@@ -908,6 +908,36 @@ fail:
         return r;
 }
 
+static int config_parse_alias(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        const char *name;
+        UnitType type;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+
+        name = basename(filename);
+        type = unit_name_to_type(name);
+        if (!unit_type_may_alias(type))
+                return log_syntax(unit, LOG_WARNING, filename, line, 0,
+                                  "Aliases are not allowed for %s units, ignoring.",
+                                  unit_type_to_string(type));
+
+        return config_parse_strv(unit, filename, line, section, section_line,
+                                 lvalue, ltype, rvalue, data, userdata);
+}
+
 static int config_parse_also(
                 const char *unit,
                 const char *filename,
@@ -993,7 +1023,7 @@ static int unit_file_load(
                 SearchFlags flags) {
 
         const ConfigTableItem items[] = {
-                { "Install", "Alias",           config_parse_strv,             0, &info->aliases           },
+                { "Install", "Alias",           config_parse_alias,            0, &info->aliases           },
                 { "Install", "WantedBy",        config_parse_strv,             0, &info->wanted_by         },
                 { "Install", "RequiredBy",      config_parse_strv,             0, &info->required_by       },
                 { "Install", "DefaultInstance", config_parse_default_instance, 0, info                     },