INT 21,50 - Set Current Process ID (Undocumented DOS 2.x)

	AH = 50h
	BX = process ID number (process PSP segment address)

	returns nothing

	- the process ID number is actually the segment address of a
	  program's PSP.  This is useful for TSR's to access their own
	  file handle table inside their PSP.
	- this function cannot be called while in an INT 28 handler in
	  DOS 2.x unless the critical error flag is set or stack corruption
	  will occur
	- safe in DOS 3.x INT 28 handlers since they use a different stack
	  by default
	- available in OS/2 compatibility box
	- see  INT 21,51   INT 21,62