Support 3rd party installer files for external programs (doors)

By simply installing a supported 3rd party door game into your
"xtrn" directory, install-xtrn.js or xtrn-setup.js can automatically
discover and configure your newly installed door for Synchronet. Easy peasy.

Resolves issue #181 (great idea) by Michael Long. Deuce and others have had
similar ideas in the past.

Now, we don't have to rely on the authors of potentially-abandoned door games
to create and include a Synchronet install-xtrn.ini file. We can just do that
for ourselves and make installing all (known) door games super easy.

Many more xtrn/3rdp-install/*.ini files will be forthcoming!