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
(cherry picked from commit 42664610795bc0a728851ba6223fcf9b93801167)
Add a manpage for it, assign the copyright to the OpenBSD project on it since it
is mostly copy/paste from OpenBSD manpage.
style(9) fixes
Differential Revision: https://reviews.freebsd.org/D2420
Reviewed by: kib
