Fix removal of file locks on MS-Windows 9X

Fix removal of file locks on MS-Windows 9X

A new Windows-specific function for validating process ID
is introduced that does TRT with possibly negative PID
values returned by 'getpid' on Windows 9X and also with
values larger than INT_MAX, since PID on Windows is
actually an unsigned 32-bit value.
* src/w32proc.c (w32_valid_process_id): New function.
* src/w32common.h (VALID_PROCESS_ID): Define for WINDOWSNT.
* src/filelock.c [WINDOWSNT]: Include w32common.h.  (Bug#71477)