* wincap.h (wincaps::has_buggy_thread_startup): Declare.

* wincap.h (wincaps::has_buggy_thread_startup): Declare.
(wincapc::has_buggy_thread_startup): Ditto.
* wincap.cc::wincap_*): Accommodate has_buggy_thread_startup.
(wincapc::init): Explicitly turn off has_buggy_thread_startup if not WOW64.
* cygthread.h (cygthread::thread_handle): Declare/define new method.
* dcrt0.cc (_dll_crt0): Don't call __sinit here.
(dll_crt0_0): Don't call sigproc_init during initialization if
wincap.has_buggy_thread_startup().
(dll_crt0_1): Defer sigproc_init to here when
wincap.has_buggy_thread_startup().  Call __sinit after we've determined that
we're not forking.
(__main): Rework comments.  Add potential future reminder.