sysusers: allow uid:gid in sysusers.conf files

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