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/systemcmds/param/param.c

@@ -202,7 +202,7 @@ static int
 do_save(const char *param_file_name)
 {
 	/* create the file */
-	int fd = open(param_file_name, O_WRONLY | O_CREAT, 0x777);
+	int fd = open(param_file_name, O_WRONLY | O_CREAT, PX4_O_MODE_666);
 
 	if (fd < 0) {
 		warn("opening '%s' failed", param_file_name);