passwd(1): Allow password changes for PAM users

* passwd(1) previously only allowed changes for local or NIS users, which
  doesn't make sense

* Fixes FreeBSD PR bin/59638
  http://www.freebsd.org/cgi/query-pr.cgi?pr=59638

* Tested with a DragonFly (password change) and a Debian system (login)
netinet/in - Fix yet another NULL pointer panic.

DragonFly-bug: <http://bugs.dragonflybsd.org/issues/2662>
Submitted-by: Vasily Postnicov

This commit closes #2662