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:		187189
Reported by:	mcdouga9@egr.msu.edu
Tested by:	mcdouga9@egr.msu.edu
Patch by:	Marc de la Gueronniere


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273791 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f