Add zdopen(3) to complement zopen(3).

zdopen() can be used in capability mode. Update zopen.3 accordingly
and fix some grammar nits while I'm here.

Reviewed by: delphij
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18456

(cherry picked from commit 8e2a46c8bd361395add649440758880c1e3da3ee)