Put a complete set of pppd(8) sample configuration files in

Put a complete set of pppd(8) sample configuration files in
/usr/share/examples/pppd.

Update pppd(8) documentation to reflect this, usr.sbin/pppd/pppd.8.

Remove the out-of-place pppd(8) configuration files in etc/ppp,
ppp.shells.sample and ppp.deny.

Make the appropriate changes to the build process, etc/Makefile and
etc/mtree/BSD.usr.mtree, so it all works.

The files from etc/ppp, ppp.shells.sample and ppp.deny, were moved
with a repo copy. Note it in the logs with a forced commit to these
two.

Submitted by:	Maxim Konovalov <maxim@macomnet.ru> provided the new samples.