libmisc/salt.c: Use secure system ressources to obtain random bytes.

libmisc/salt.c: Use secure system ressources to obtain random bytes.

In a previous commit we introduced /dev/urandom as a source to obtain
random bytes from.  This may not be available on all systems, or when
operating inside of a chroot.

Almost all systems provide functions to obtain random bytes from
secure system ressources.  Thus we should prefer to use these, and
fall back to /dev/urandom, if there is no such function present, as
a last resort.

Signed-off-by: Björn Esser <besser82@fedoraproject.org>