review of an update to "struct export_args"

[Rick Macklem]

Hi,

I have put a patch up on phabricator
https://reviews.freebsd.org/D25088

I have listed kib@ and freqlabs@ as reviewers, but if anyone else
wishes to review it, be my guest.

It updates "struct export_args" to make the ex_flags field 64bits and
the mapped user (is called ex_anon in the current structure) is no longer
limited to 16 additional groups.

rick