[pptp-server] New feature: libwrap support

[David Luyer]

I've put optional libwrap (tcp_wrappers) support into the cvs.  Run configure
with --with-libwrap if you want it, and then configure access control as per
normal for tcp_wrappers, in /etc/hosts.allow and /etc/hosts.deny (or elsewhere
if you have a strange libwrap).

Also, the latest CVS contains the fix I mentioned earlier for making sure
the pppd's die when they should.

David.