into one if a system has an awesome getentropy().  In that case it
is valid to totally throw away the rsx state in the child.  If the
getentropy() is not very good and has a lazy reseed operation, this
combining is a bad idea, and the reseed should probably continue to
use the "something old, something new" mix.  _rs_allocate() can
accomodate either method, but not on the fly.
ok matthew

1. Use "len" parameter instead of sizeof(*rs).
2. Simplify the atfork handler to be strictly async signal safe by
simply writing to a global volatile sig_atomic_t object, and then
checking for this in _rs_forkdetect().  (Idea from discussions with
Szabolcs Nagy and Rich Felker.)
3. Use memset(rs, 0, sizeof(*rs)) to match OpenBSD's MAP_INHERIT_ZERO
fork semantics to avoid any skew in behavior across platforms.
ok deraadt

making it much easier for libressl -portable to fill in the gaps.
ok bcook beck