Add reasonable default device names for divecomputer import

So far we hard coded /dev/ttyUSB0 - which is a good starting point in
Linux but not so useful on Windows or MacOS. This was now moved into one
of our OS helper functions with (somewhat) reasonable defaults.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

linux.c

@@ -46,3 +46,8 @@ void subsurface_close_conf(void)
 {
 	/* this is a no-op */
 }
+
+const char *subsurface_USB_name()
+{
+	return("/dev/ttyUSB0");
+}