Bug 4073: Cygwin compile errors

Bug 4073: Cygwin compile errors

Remove the definition of _SQUID_WINDOWS_ for Cygwin builds. The blend
of win32 and Linux environments is sufficiently different to have major
build issues. We have a precedent in kFreeBSD blend of BSD and Linux to
consider Cygwin a blend and first-class OS.

Also, temporarily disable the Win32-specific libraries and objects until
they can be properly tested.

Fix some small remaining compile errors after the above.

 Cygwin Windows build is sponsored by Diladele B.V.