[krbdev.mit.edu #3424] Add PAM support to ftpd

[nate.yocom@centrify.com via RT]

Users without kerberos credentials or local accounts (i.e. PAM users)
cannot login using ftp.  The attached patch adds a new function to
verify user credentials via PAM, and includes the use of that function
to effect login if  the kerberos or the pam check come back successful.
None of this code is included unless HAVE_PAM is set - patch includes
changes to detect the existence of pam libraries/headers.

Nate Yocom
Senior Software Engineer
Centrify Corporation
425.462.5894
www.centrify.com