sysusers: insist that root group is 0

sysusers: insist that root group is 0

In https://bugzilla.redhat.com/show_bug.cgi?id=2156900 sysusers was reporting a
conflict between the following lines:
  u root 0:0 "Super User" /root /bin/bash
  u root    0     "Super User" /root

The problem is that those configurations are indeed not equivalent. If group 0
exists with a different name, the first line would just create the user, but the
second line would create a 'root' group with a different GID. The second
behaviour seems definitely wrong. (Or at least more confusing in practice than
the first one. The system is in a strange shape, but the second approach takes
an additional step than is worse than doing nothing.)

When this line was initially added, we didn't have the uid:gid functionality for
'u', so we didn't think about this too much. But now we do, so we should use it.

$ build/systemd-sysusers --root=/var/tmp/inst7 --inline 'g foobar 0'
Creating group 'foobar' with GID 0.

$ build/systemd-sysusers --root=/var/tmp/inst7 --inline 'u root 0 "Zuper zuper"'
src/sysusers/sysusers.c:1365: Creating group 'root' with GID 999.
src/sysusers/sysusers.c:1115: Suggested user ID 0 for root already used.
src/sysusers/sysusers.c:1183: Creating user 'root' (Zuper zuper) with UID 999 and GID 999.

vs.

$ build/systemd-sysusers --root=/var/tmp/inst7 --inline 'u root 0:0 "Zuper zuper"'
src/sysusers/sysusers.c:1183: Creating user 'root' (Zuper zuper) with UID 0 and GID 0.