pw: Use existing group entry, even if it already has members
Fix the following problem:
- A nonexistent user, someuser, is added to /etc/group as someuser:*:12345:someuser.
- someuser is then created with the default login group.
A second group entry for someuser will be created.
someuser:*:12345:someuser someuser:*:12346:
With this fix, the existing group entry will be used.
PR: 238995
Reviewed by: bapt, jrm
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41057