Unlike OpenBSD's, our setusercontext() will intentionally ignore the user's
own umask setting (from ~/.login.conf) unless running with the user's UID.
Therefore, we need to call it again with LOGIN_SETUMASK after changing UID.

PR:		bin/176740
Submitted by:	John Marshall <john.marshall@riverwillow.com.au>
MFC after:	1 week