implemented TALK, LISTEN, UNTLK and UNLSN routines

c64/kernal/listen.s

@@ -1,3 +1,7 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
+
 listen:
-	rts
+
+	;; This trivial routine is documented in 'Compute's Mapping the Commodore 64', page 223
+	ora $20
+	jmp iec_tx_byte

c64/kernal/talk.s

@@ -1,3 +1,7 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
+
 talk:
-	rts
+
+	;; This trivial routine is documented in 'Compute's Mapping the Commodore 64', page 223
+	ora $40
+	jmp iec_tx_byte

c64/kernal/unlsn.s

@@ -1,3 +1,7 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
+
 unlsn:
-	rts
+
+	;; This trivial routine is documented in 'Compute's Mapping the Commodore 64', page 224
+	lda $5F
+	jmp iec_tx_byte

c64/kernal/untlk.s

@@ -1,3 +1,7 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
+
 untlk:
-	rts
+
+	;; This trivial routine is documented in 'Compute's Mapping the Commodore 64', page 224
+	lda $5F
+	jmp iec_tx_byte