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.


(gutteridge)

$NetBSD: README.pkgsrc,v 1.1 2021/05/03 23:31:40 gutteridge Exp $

If xscreensaver is built with the non-default "pam" option, on NetBSD, the
"pam_pwauth_suid.so" module will be used to authenticate against a shadow
password database. Note that use of this module might allow programs with
your privileges to get a copy of your plaintext password as typed in for
unlocking. In order to make unlocking work, you need to add an xscreensaver
file to your PAM configuration directory (usually /etc/pam.d). On NetBSD,
you can find a sample pam.d file for xscreensaver in:
	${PREFIX}/share/examples/pam-pwauth_suid

By default (if the "pam" option isn't set), pkgsrc will set the
	${PREFIX}/bin/xscreensaver
executable setuid root. Since this is a relatively complex program,
there is the risk of other exploits.