AP_HAL: add O_CLOEXEC in places missing it

By opening with O_CLOEXEC we make sure we don't leak the file descriptor
when we are exec'ing or calling out subprograms. Right now we currently
don't do it so there's no harm, but it's good practice in Linux to have
it.

libraries/AP_HAL/utility/dsm.cpp

@@ -464,7 +464,7 @@ static uint64_t micros64(void)
 
 int main(int argc, const char *argv[])
 {
-    int fd = open(argv[1], O_RDONLY);
+    int fd = open(argv[1], O_RDONLY|O_CLOEXEC);
     if (fd == -1) {
         perror(argv[1]);
         exit(1);

libraries/AP_HAL/utility/srxl.cpp

@@ -177,7 +177,7 @@ static uint64_t micros64(void)
 
 int main(int argc, const char *argv[])
 {
-    int fd = open(argv[1], O_RDONLY);
+    int fd = open(argv[1], O_RDONLY|O_CLOEXEC);
     if (fd == -1) {
         perror(argv[1]);
         exit(1);