Windows: fix code wrappers for Cygwin and generic Win32 code

Windows: fix code wrappers for Cygwin and generic Win32 code

_SQUID_WIN32_ was being used to wrap all code built specific for Windows
but with any compiler on that platform.
 - rename to _SQUID_WINDOWS_ to match OS wrpper naming convention.
 - compact several macros using verbose test: (cygwin or Ming or any)

Cleans up all affected tests to match current precompiler code style.

Also cleans up all tests involving _SQUID_CYGWN_ to match code styles.