Change xgetpwnamid/xgetgrnamid to xgetuid/xgetgid returning the id number

Change xgetpwnamid/xgetgrnamid to xgetuid/xgetgid returning the id number
instead of a struct. This means it can return "12345" even if that user/group
doesn't exist in /etc/passwd and similar.

All the users were immediately dereferencing it to get pw_uid or gr_gid
anyway, so just return it directly and adjust the users. This fixes
things like "chown 12345:23456 filename".