Make sure IOV_MAX is defined.

Make sure IOV_MAX is defined.

We stopped defining IOV_MAX on non-Windows systems in 75357ab94, on
the assumption that every non-Windows system defines it in <limits.h>
as required by X/Open.  GNU Hurd, however, doesn't follow that
standard either.  Put back the old logic to assume 16 if it's
not defined.

Author: Michael Banck <mbanck@gmx.net>
Co-authored-by: Christoph Berg <myon@debian.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/6862e8d1.050a0220.194b8d.76fa@mx.google.com
Discussion: https://postgr.es/m/6846e0c3.df0a0220.39ef9b.c60e@mx.google.com
Backpatch-through: 16