pam_message array contains exactly one PAM_PROMPT_ECHO_OFF message.
Instead, deal with however many messages there are, and don't throw error
for PAM_ERROR_MSG and PAM_TEXT_INFO messages.  This logic is borrowed from
openssh 5.2p1, which hopefully has seen more real-world PAM usage than we
have.  Per bug #5121 from Ryan Douglas, which turned out to be caused by
the conv_proc being called with zero messages.  Apparently that is normal
behavior given the combination of Linux pam_krb5 with MS Active Directory
as the domain controller.

Patch all the way back, since this code has been essentially untouched
since 7.4.  (Surprising we've not heard complaints before.)