Many changes in latest snapshot

[DJ Delorie]

> I've also eliminated the USE_REGISTRY_OPTIONS conditional.  cygwin will
> now search the registry for CYGWIN-environment-variable-like options.

Lesson from DJGPP: add a way to say that program A should use whatever
program B has (recursively).  Example: if A's entry says "=B" it
should suck in what B has.  Example: "ed" -> "=vi" where "vi" ->
"notty,ntea".