pam_shells:
	Authentication is granted if the users shell is listed in
	/etc/shells. If no shell is in /etc/passwd (empty), the
	/bin/sh is used (following ftpd's convention).

	Also checks to make sure that /etc/shells is a plain
	file and not world writable.

	- Erik Troan <ewt@redhat.com>, Red Hat Software.
		August 5, 1996.