implemented TALK/LISTEN/UNTLK/UNLSN in the internal DOS

src/dos_m65/,aliases_dos.s

@@ -7,19 +7,19 @@
 ; - additional 16KB RAM is reserved for DOS too ($14000-$17FFF), and can be banked-in if needed
 
 
-
 	;
-	; Page 0
+	; Page start+0
 	;
 
 	!addr MAGICSTR         = $8000 ; magic string; if not matching, DOS considered non-functional
 
 	; General DOS configuration
 
-	!addr UNIT_SDCARD      = $8005
-	!addr UNIT_FLOPPY      = $8006
-	!addr UNIT_RAMDISK     = $8007
+	!addr UNIT_SDCARD      = $8005 ; unit number for device, 0 = none
+	!addr UNIT_FLOPPY      = $8006 ; as above
+	!addr UNIT_RAMDISK     = $8007 ; as above
 
 	; General DOS status
 
-	!addr UNIT_HANDLER     = $8008
+	!addr IDX_LISTENER     = $8008 ; listener idx * 2, >=$80 = none
+	!addr IDX_TALKER       = $8009 ; talker   idx * 2, >=$80 = none