Integrate usr.sbin/useradd/t_useradd.sh from NetBSD into FreeBSD as pw_test

- Suffix useradd/userdel commands with pw
- Remove the atf_expect_fail for bin/39546

Phabric: D921
Reviewed by: brd
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274011 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f