INT 15 - MultiDOS Plus - READ MAILBOX                                           
	AH = 06h
	AL = mailbox number (00h-3Fh)
	CX = size of buffer in bytes
	ES:DI -> buffer for message
Return: AH = status
	    00h successful
		CX = number of bytes copied
		DX = actual length of message
	    02h invalid mailbox number
Note:	if the caller's buffer is not large enough, the message is truncated
	  and the remainder is lost
SeeAlso: AH=04h"MultiDOS",AH=05h"MultiDOS"