util: fall back to reading /dev/urandom when getrandom() blocks

With recent changes in the Linux kernel, the getrandom() system call may
block for a long time after boot on machines that don't have enough
entropy. It blocks the chronyd's initialization before it can detach
from the terminal and may cause a chronyd service to fail to start due
to a timeout.

At least for now, enable the GRND_NONBLOCK flag to make the system call
non-blocking and let the code fall back to reading /dev/urandom (which
never blocks) if the system call failed with EAGAIN or any other error.

This makes the start of chronyd non-deterministic with respect to files
that it needs to open and possibly also makes it slightly easier to
guess the transmit/receive timestamp in client requests until the
urandom source is fully initialized.

util.c

@@ -1224,7 +1224,7 @@ get_random_bytes_getrandom(char *buf, unsigned int len)
       if (disabled)
         break;
 
-      if (getrandom(rand_buf, sizeof (rand_buf), 0) != sizeof (rand_buf)) {
+      if (getrandom(rand_buf, sizeof (rand_buf), GRND_NONBLOCK) != sizeof (rand_buf)) {
         disabled = 1;
         break;
       }