MFC: 272445,272578,273772,273779,273782,273786,273787,273791

Add a test for bug 191427 where pw(8) will go into an infinite loop
Add some tests for modifying groups
When a group is renamed then the group has been invalidated for sure.
In that case get the group information using the new name.

Fix a regression in pw usermod -G list

The user was perperly adding the to different groups from "list" but was not
removed from the other groups it could have belong to.

Do not delete the group wheel when bad argument is passed to pw groupdel -g

Check that the -g argument is actually a number, if not report an error.
This argument is converted without checking with atoi(3) later so without this
check it converts any alpha entries into 0 meaning it deletes the group wheel

Ensure pw userdel -u <invalid> do not try to remove root

Check the uid passed is actually a number as early as possible

Fix renaming a group via the gr_copy function

Add a regression test to pw(8) because the bug was discovered via using:
pw groupmod

PR:		193704 [1], 185666 [2], 90114 [3], 187189 [4]
Submitted by:	Marc de la Gueronniere [4]
Reported by:	az [1], sub.mesa@gmail.com [2], bkoenig@cs.tu-berlin.de [3],
		mcdouga9@egr.msu.edu [4]