usermod: respect --prefix for --gid option

usermod: respect --prefix for --gid option

The --gid option accepts a group name or id. When a name is provided, it
is resolved to an id by looking up the name in the group database
(/etc/group).

The --prefix option overides the location of the passwd and group
databases. I suspect the --gid option was overlooked when wiring up the
--prefix option.

useradd --gid already respects --prefix; this change makes usermod
behave the same way.

Fixes: b6b2c756c91806b1c3e150ea0ee4721c6cdaf9d0
Signed-off-by: Mike Gilbert <floppym@gentoo.org>