util: add function to generate random bytes

Add a function to fill a buffer with random bytes which uses a better
PRNG than random(). Use arc4random() if it's available on the system.
Fall back to reading from /dev/urandom, which should be available on
all currently supported systems.

configure

@@ -567,6 +567,10 @@ then
   MYCFLAGS="$MYCFLAGS -pthread"
 fi
 
+if test_code 'arc4random_buf()' 'stdlib.h' '' '' 'arc4random_buf(NULL, 0);'; then
+  add_def HAVE_ARC4RANDOM
+fi
+
 timepps_h=""
 if [ $feat_refclock = "1" ] && [ $feat_pps = "1" ]; then
   if test_code '<sys/timepps.h>' 'sys/timepps.h' '' '' ''; then