PR gdb/14018 -- avoid "PC register not available" errors.

gdb/windows-nat.c (thread_rec): Don't display a warning when
SuspendThread fails with ERROR_ACCESS_DENIED.  If SuspendThread
fails for any reason, set th->suspended to -1, so that we don't
try to resume such a thread.  Also, don't return NULL in these
cases, to avoid completely ruin the session due to "PC register is
not available" error.
(do_windows_fetch_inferior_registers): Check errors in
GetThreadContext call.
(windows_continue): Accept an additional argument KILLED; if not
zero, ignore errors in the SetThreadContext call, since the
inferior was killed and is shutting down.
(windows_resume, get_windows_debug_event)
(windows_create_inferior, windows_mourn_inferior)
(windows_kill_inferior): All callers of windows_continue changed
to adjust to its new calling sequence.