lib/libc: add recallocarray()

This function from OpenBSD is a hybrid of reallocarray() and calloc().
It reallocates an array, clearing any newly allocated items.
reallocarray() ultimately originates from OpenBSD.

The source is taken from lib/libopenbsd, which now no longer has the
function unless when bootstrapping (needed for mandoc).

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D52863