Throughout, revert ill-conceived replacement of hMainThread with

GetCurrentThread/NtCurrentThread.
	* dcrt0.cc (dll_crt0_0): Duplicate main thread handle to hMainThread
	again.

winsup/cygwin/globals.cc

@@ -18,6 +18,7 @@ details. */
 #include <malloc.h>
 #include <cygwin/version.h>
 
+HANDLE NO_COPY hMainThread;
 HANDLE NO_COPY hProcToken;
 HANDLE NO_COPY hProcImpToken;
 HMODULE NO_COPY cygwin_hmodule;