Prevent excessively long username going to PAM.

This is a mitigation for a buffer overflow in Solaris' PAM username
handling (CVE-2020-14871), and is only enabled for Sun-derived PAM
implementations.  This is not a problem in sshd itself, it only
prevents sshd from being used as a vector to attack Solaris' PAM.
It does not prevent the bug in PAM from being exploited via some other
PAM application.

Based on github PR#212 from Mike Scott but implemented slightly
differently.  ok tim@ djm@
