randread: use /dev/urandom to seed, not just getpid etc

randread: use /dev/urandom to seed, not just getpid etc

* gl/lib/rand-isaac.c (isaac_seed_start): New arg SEEDED.
(isaac_seed): New args FD and BYTES_BOUND.  Read from FD if possible.
Don't bother with low-quality sources if FD has enough bytes.
* gl/lib/rand-isaac.h: New size_t arg for isaac_seed.
* gl/lib/randread.c: Include fcntl.h, unistd.h.
(NAME_OF_NONCE_DEVICE): New #define.
(nonce_device): New static var.
(randread_new): Use nonce device if available.