Switch random_get_pseudo_bytes() shim to arc4rand().

Our shim for Solaris random_get_bytes() uses read_random(), that looks
reasonable, since it guaranties reliably seeded random data. On the other
side Solaris random_get_pseudo_bytes() does not provide this guarantie,
and its original Solaris implementation is equivalent to our arc4rand(),
using software crypto without stressing slower hardware RNG.