config: Add API to get/set config variables.

It is useful to be able to set variables through an API provided by
the config module; this means that it is possible for one module to
set config variables of another in a more loosely-coupled way.
Set GUS patch path variable using D_SetVariable.

commit 42faefce1fd03 added code to set the GUS patch path
automatically when the BFG Edition is installed, but this caused
problems because gusconf.c is not included as part of the build for
the setup tool. Use D_SetVariable() instead which accomplishes the
same thing without a hard dependency.
More robust check for auto-loaded DEHACKED lumps.
Indicate keys and buttons that require special features enabled in setup.
Much more conservative default settings.
Merge branch 'master' of https://github.com/chocolate-doom/chocolate-doom