filter all passwords (not only changed) from periodic passwd backup

The periodic 200.backup-passwd script outputs any differences it finds
in master.passwd, relative to the previous backup. It intends to elide
the encrypted password field, but previously did so only for changed
lines (i.e., those beginning with - or + in the diff).

Apply the sed expression also to unchanged lines to also elide their
passwords.

PR: 223461
Reported by: Andre Albsmeier
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation