INT 15 - MultiDOS Plus - CHECK MAILBOX                                          
	AH = 05h
	AL = mailbox number (00h-3Fh)
Return: AH = status
	    00h successful
		DX = length of first message in queue, 0000h if no message
	    02h invalid mailbox number
SeeAlso: AH=04h"MultiDOS",AH=06h"MultiDOS"