Modify mountd to use the new struct export_args committed by r362158.

r362158 modified struct export_args for make the ex_flags field 64bits
and also changed the anonymous credentials to allow more than 16 groups.
This patch fixes mountd.c to use the new structure.
It does allocate larger exportlist and grouplist structures now.
That will be fixed in a future commit.
The only visible change will be that the credentials provided for the
-maproot and -mapall exports options can now have more than 16 groups.

Reviewed by: kib, freqlabs
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D25088