Remove usage of find_inferior in win32-i386-low.c
Straightforward replacement of find_inferior with the overload of
for_each_thread that filters on pid. I am able to build-test this
patch, but not run it.
gdb/gdbserver/ChangeLog:
* win32-i386-low.c (update_debug_registers_callback): Rename
to ...
(update_debug_registers): ... this, return void, remove pid_p arg.
(x86_dr_low_set_addr, x86_dr_low_set_control): Use for_each_thread.