varlink: also honour new env var $SYSTEMD_VARLINK_LISTEN in varlink_server_listen_auto()

varlink_server_listen_auto() is supposed to be the one-stop solution for
turning simple command line tools into IPC services. They aren't easy to
test/debug however, since you have to invoke them through a service
manager.

Let's make this easier: if the SYSTEMD_VARLINK_LISTEN env var is set,
let's listen on the socket specified therein. This makes things easier
to gdb: just run the service from the cmdline.

docs/ENVIRONMENT.md

@@ -616,3 +616,8 @@ Tools using the Varlink protocol (such as `varlinkctl`) or sd-bus (such as
 
 * `$SYSTEMD_SSH` – the ssh binary to invoke when the `ssh:` transport is
   used. May be a filename (which is searched for in `$PATH`) or absolute path.
+
+* `$SYSTEMD_VARLINK_LISTEN` – interpreted by some tools that provide a Varlink
+  service. Takes a file system path: if specified the tool will listen on an
+  `AF_UNIX` stream socket on the specified path in addition to whatever else it
+  would listen on.

src/shared/varlink.c

@@ -3477,6 +3477,14 @@ int varlink_server_listen_auto(VarlinkServer *s) {
                 n++;
         }
 
+        /* For debug purposes let's listen on an explicitly specified address */
+        const char *e = secure_getenv("SYSTEMD_VARLINK_LISTEN");
+        if (e) {
+                r = varlink_server_listen_address(s, e, FLAGS_SET(s->flags, VARLINK_SERVER_ROOT_ONLY) ? 0600 : 0666);
+                if (r < 0)
+                        return r;
+        }
+
         return n;
 }
 
@@ -3922,6 +3930,10 @@ int varlink_invocation(VarlinkInvocationFlags flags) {
 
         /* Returns true if this is a "pure" varlink server invocation, i.e. with one fd passed. */
 
+        const char *e = secure_getenv("SYSTEMD_VARLINK_LISTEN"); /* Permit a manual override for testing purposes */
+        if (e)
+                return true;
+
         r = sd_listen_fds_with_names(/* unset_environment= */ false, &names);
         if (r < 0)
                 return r;