Throughout, replace hMainProc with GetCurrentProcess/NtCurrentProcess

	Throughout, replace hMainProc with GetCurrentProcess/NtCurrentProcess
	according to context.  Throughout, replace hMainThread with
	GetCurrentThread/NtCurrentThread according to context.
	* dcrt0.cc (dll_crt0_0): Drop duplication of GetCurrentProcess to
	hMainProc.  Drop duplication of GetCurrentThread to hMainThread.
	* dtable.cc (dtable::stdio_init): Remove useless comment.
	* globals.cc (hMainProc): Remove.
	(hMainThread): Remove.
	* ntdll.h (NtCurrentProcess): Define.
	(NtCurrentThread: Define.