splitting dependencies in cygwin1.dll

[Robert Collins]

Chris,
	I'd like to split out some of the classes & headers in
cygwin1.dll to make it more loosely coupled. This wouldn't make it less
able to use inlined calls, but would reduce recompiles where non-related
things have not actually changed.

I'd also like to make the headers consistently forward declare required
classes / include their own pre-requisite headers (with forward
declarations preferred).

I'll run all such changes through cygwin-patches for review, but first:
do you have any objections to this being done?

Rob