pam_krb5: Fix manual page in MIT case

• Always install the manual page as pam_krb5(8) regardless of which version we're using.
• Regenerate it using pod2mdoc instead of pod2man so it doesn't claim to be part of “User Contributed Perl Documentation”.
• Put the correct section number in the header and footer.
• Don't cross-reference non-existent pam(7) manual page.

Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D53885

(cherry picked from commit 961b934407f09d4241cfad76a9c8833baa2650e1)