Log Message:
xscreensaver: adjust some documentation

Move the existing MESSAGE content to README.pkgsrc, as much of it
pertains to a non-default option that most users don't need to consider,
and otherwise is not a call to action for a user to do anything. While
here, restructure it a bit.

Also remove the sample NetBSD PAM configration file here, as the
security/pam-pwauth_suid package provides its own (slightly newer)
version of the same, and has its own MESSAGE that references its version
and its distinct installation location (and is a call to action that
users very much need to know about, as otherwise they'll end up with an
X session they can't unlock). There's no point possibly confusing people
with different example files. This addresses PR pkg/50622.