sysusers: allow uid:gid in sysusers.conf files

This PR allows to write sysuser.conf lines like:
```
u games 5:60 -
```
This will create an a "games" user with uid 5 and games group with
gid 60. This is arguable ugly, however it is required to represent
certain configurations like the default passwd file on Debian and
Ubuntu.

When the ":" syntax is used and there is a group with the given
gid already then no new group is created. This allows writing the
following:
```
g unrelated 60
u games 5:60 -
```
which will create a "games" user with the uid 5 and the primary
gid 60. No group games is created here (might be useful for [1]).

[1] https://pagure.io/packaging-committee/issue/442

man/sysusers.d.xml

@@ -191,7 +191,10 @@ u      root   0               "Superuser"           /root</programlisting>
       in the file system. In this case, the UID/GID is read from the
       path's owner/group. This is useful to create users whose UID/GID
       match the owners of pre-existing files (such as SUID or SGID
-      binaries).</para>
+      binaries).
+      The syntax <literal><replaceable>uid</replaceable>:<replaceable>gid</replaceable></literal> is also supported to
+      allow creating user and group pairs with different numeric UID and GID values. If a group with the indicated GID already exists it is not created.
+      </para>
 
       <para>For <varname>m</varname> lines, this field should contain
       the group name to add to a user to.</para>