sysusers: check if requested group name matches user name in queue

When creating a user, check if the requested group name matches a user
name in the queue. If that matched user name is also going to be a group
name, then use it for the new user too. In other words, allow the
following:

 u foo -
 u bar -:foo

when both foo and bar are new users.

Fixes #33547

test/test-sysusers/test-16.input

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+#
+# Test fix for https://github.com/systemd/systemd/issues/33547.
+#
+#Type Name ID
+u     foo  -
+u     bar  300:foo