sysusers: Provide meson argument to set gid for 'users' group (#7533)

To allow better integration with distributions requiring an explicitly
set gid for the `users` group, provide the new `-Dusers-gid` option to
set to a new numeric value.

In the absence of a specified gid, we'll fallback to the default existing
behaviour of `-` as the gid value, to automatically assign the next available
gid on the system.

sysusers.d/basic.conf.in

@@ -35,4 +35,4 @@ g tape    -     -            -
 g video   -     -            -
 
 # Default group for normal users
-g users   -     -            -
+g users   @USERS_GID@     -            -