UART manage /DTR along with /RTS

The RC2014 Single and Dual UART Modules do not have the /RTS line connected. The /DTR line is used to signal flow control.

This PR simply adds active management of the /DTR.
The current UART initialisation process already does configuration, but not active management.

Source/HBIOS/uart.asm

@@ -320,7 +320,7 @@ UART_INTRCV1:
 	LD	C,(IY+3)		; LSR PORT TO C
 	DEC	C			; POINT TO MCR PORT
 	IN	A,(C)			; GET MCR VALUE
-	AND	~%00000010		; CLEAR RTS
+	AND	~%00000011		; CLEAR RTS & DTR
 	OUT	(C),A			; AND SAVE IT
 ;	
 UART_INTRCV2:
@@ -412,7 +412,7 @@ UART_INTIN:
 	LD	C,(IY+3)		; LSR PORT TO C
 	DEC	C			; POINT TO MCR PORT
 	IN	A,(C)			; GET MCR VALUE
-	OR	%00000010		; SET RTS
+	OR	%00000011		; SET RTS & DTR
 	OUT	(C),A			; AND SAVE IT
 ;
 UART_INTIN1: