Change type of argument to windows-nat.c:thread_rec

Change type of argument to windows-nat.c:thread_rec

windows-nat.c:thread_rec accepts an integer parameter whose
interpretation depends on whether it is less than, equal to, or
greater than zero.  I found this confusing at times, so this patch
replaces it with an enum instead.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (enum thread_disposition_type): New.
	(thread_rec): Replace "get_context" parameter with "disposition";
	change type.
	(windows_add_thread, windows_nat_target::fetch_registers)
	(windows_nat_target::store_registers, handle_exception)
	(windows_nat_target::resume, get_windows_debug_event)
	(windows_nat_target::get_tib_address)
	(windows_nat_target::thread_name)
	(windows_nat_target::thread_alive): Update.