Add a manual page for iflib.9 so that man iflib is useful

[benno]

Summary

Documentation for iflib exists in iflibdi.9, iflibdd.9 and iflibtxrx.9 but there's no overarching document which means that man iflib doesn't do much that's useful. man -k iflib does but this adds an iflib.9 so people will at least find something when they do man iflib.

[shurd]

Looks good to me, waiting on @manpages approval.