Fixed passed ot open() for O_CREAT

In nuttx the mode parameter to open is not required but in Linux,
and per the POSIX spec, mode is required if the O_CREAT flag is
passed.

The mode flags are different for NuttX and Linux so a new set of
PX4 defines was added:

PX4_O_MODE_777 - read, write, execute for user, group and other
PX4_O_MODE_666 - read, and write for user, group and other
PX4_O_MODE_600 - read, and write for user

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/systemlib/param/param.c

@@ -728,7 +728,7 @@ param_save_default(void)
 	const char *filename = param_get_default_file();
 
 	/* write parameters to temp file */
-	fd = PARAM_OPEN(filename, O_WRONLY | O_CREAT, 0x777);
+	fd = PARAM_OPEN(filename, O_WRONLY | O_CREAT, PX4_O_MODE_666);
 
 	if (fd < 0) {
 		warn("failed to open param file: %s", filename);