freebsd-update: Update /etc/passwd after password db changes

Add -p to pwd_mkdb in order to ensure password db changes are also
included in /etc/passwd.

PR: 165954, 232921, 229487
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by: jilles
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18827