backup-passwd: mask out all passwords in the diff

The previous expression borked if a username had a plus or hyphen in it.
This is needlessly restrictive- at leSt a hyphen in the middle is valid.
Instead of playing this game, let's just assume the username can't contain a
colon and mask out the second field.

Submitted by: sigsys gmail com
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D23548