MFV r319948: 5428 provide fts(), reallocarray(), and strtonum()

MFV r319948: 5428 provide fts(), reallocarray(), and strtonum()

illumos/illumos-gate@4585130b259133a26efae68275dbe56b08366deb
https://github.com/illumos/illumos-gate/commit/4585130b259133a26efae68275dbe56b08366deb

https://www.illumos.org/issues/5428

Most of the upstream change is not applicable to FreeBSD.
Only the renaming of strtonum to zfs_strtonum is relevant to us.
And we already had it partially done.

Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Joshua M. Clulow <josh@sysmgr.org>
Author: Yuri Pankov <yuri.pankov@nexenta.com>
MFC after:	1 week