INT 21,5C - Lock/Unlock File Access (DOS 3.0+)

	AH = 5C
	AL = 00  lock file
	   = 01  unlock file
	BX = file handle
	CX = most significant word of region offset
	DX = least significant word of region offset
	SI = most significant word of region length
	DI = least significant word of region length


	on return:
	AX = error code if CF set  (see DOS ERROR CODES)


	- used for networking and multi-tasking system to preserve
	  data integrity
	- duplicated handles inherit access to locked regions
	- EXEC'd programs do not inherit access to locked regions