Add a stub manual page for iflib(9).

Currently 'man -k iflib' would find you the right pages for iflib
documentation, namely iflibdd(9) and iflibdi(9) but 'man iflib' would leave
you in the dark. This allows both approaches to find the relevant
documentation.

Reviewed by: kmacy, shurd
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D15219