server: read/write now goes through connection fn's

depending on whether the connection is over a socket
or pipe, the read is done differently.

pipes can return -1 when writing 0 bytes, make 0 byte
writes a successful no-op. 0 byte writes falls out
naturally of tcl server code.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

src/server/gdb_server.c

@@ -328,7 +328,7 @@ static int gdb_write(struct connection *connection, void *data, int len)
 	if (gdb_con->closed)
 		return ERROR_SERVER_REMOTE_CLOSED;
 
-	if (write_socket(connection->fd_out, data, len) == len)
+	if (connection_write(connection, data, len) == len)
 	{
 		return ERROR_OK;
 	}