Add recallocarray(), introduced in OpenBSD 6.1, and freezero(), introduced in OpenBSD 6.2. The former is imported as-is from OpenBSD, while the latter is the non-malloc-internal branch of the same code (and also the OpenSSH portable variant).
Both of these originated in OpenBSD, but have also been implemented by IllumOS, cf. https://www.illumos.org/issues/8546
Documentation for these functions is in malloc(3) upstream, the relevant parts of which were previously imported in reallocarray(3bsd). Update reallocarray(3bsd) with the changes that were introduced since, and add the relevant bits for recallocarray() and freezero(), plus aliases.