MSC: int locking( int handle, int mode, long nbytes )

	- prototype in io.h, sys\locking.h

	- handle = opened file handle
	- nbytes = bytes to lock beginning with current file position
	- mode	= locking mode:
		= LK_LOCK	lock region; on failure waits 1 sec and
				attempts again;  tries 10 times
		= LK_RLCK	same as LK_LOCK
		= LK_NBLCK	lock region; returns immediately on error
		= LK_NBRLCK	lock region; returns immediately on error
		= LK_UNLCK	unlock previously locked region

	- multiple locks may occur in a single file
	- overlapping locked regions are not allowed
	- returns 0 on success or -1 on failure

	- Turbo C uses lock() and unlock()