* remote.c (remote_open): Use SERIAL_OPEN instead of serial_open.

	* remote.c (remote_open):  Use SERIAL_OPEN instead of serial_open.
	(putpkt, getpkt):  Use new return codes for SERIAL_READCHAR.
	* ser-go32.c:  Return -1 on most failures, 0 on most successes,
	and use new return codes for go32_readchar().
	* ser-unix.c:  Ditto.  Also, move error handling up to caller for
	SERIAL_SETBAUDRATE().
	* serial.c (serial_open):  Internal call, not SERIAL_OPEN to get
	to specific routine.
	(serial_close):  New routine to wrap around device close routine.
	serial.h:  Clean & document return values more clearly.