sshd: make getpwclass wrapper MON_ISAUTH not MON_AUTH

In r339216 a privsep wrapper was added for login_getpwclass to address
PR 231172. Unfortunately the change used the MON_AUTH flag in the
wrapper, and MON_AUTH includes MON_AUTHDECIDE which triggers an
auth_log() on each invocation. getpwclass() does not participate in the
authentication decision, so should be MON_ISAUTH instead.

PR: 234793
Submitted by: Henry Hu
Reviewed by: Yuichiro NAITO
MFC after: 1 week