gdb/windows-nat.c: Get rid of main_thread_id global

gdb/windows-nat.c: Get rid of main_thread_id global

This global is meant to point to the "main" thread of execution of
the program we are debugging. It is set when attaching to a process
or when receiving a CREATE_PROCESS_DEBUG_EVENT event. The theory at
the time was that this was also going to be the thread receiving
the EXIT_PROCESS_DEBUG_EVENT event.

Unfortunately, we have discovered since then that this is actually
not guaranteed. What this means in practice is that there is moderate
risk that main_thread_id refers to a thread which no longer exists.

This global is used in 3 situations:
  - OUTPUT_DEBUG_STRING_EVENT
  - LOAD_DLL_DEBUG_EVENT
  - UNLOAD_DLL_DEBUG_EVENT

It's not clear why we would need to use the main_thread_id in those cases
instead of using the thread ID provided by the kernel events itself.
So this patch implements this approach, which then allows us to delete
the main_thread_id global.

gdb/testsuite:

	* windows-nat.c (main_thread_id): Delete.
	(handle_output_debug_string): Replace main_thread_id by
	current_event.dwThreadId.
	(fake_create_process): Likewise.
	(get_windows_debug_event) <CREATE_PROCESS_DEBUG_EVENT>:
	Do not set main_thread_id.
	<LOAD_DLL_DEBUG_EVENT>: Replace main_thread_id by
	current_event.dwThreadId.
	<UNLOAD_DLL_DEBUG_EVENT>: Likewise.