INT 21,56 - Rename File

	AH = 56h
	DS:DX = pointer to old ASCIIZ path/filename
	ES:DI = pointer to new ASCIIZ path/filename

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

	- supports full pathnames and allows renaming files across
	  directories and in DOS 3.x allows renaming subdirectories
	- does not support use of wildcards unless invoked from via
	  INT 21,5D in which case error code 12h is returned
	- unpredictable result may occur if an opened file is renamed
	- see Bibliography reference to "Undocumented DOS"