adding "-manage-gids" option to the NFS server

[Rick Macklem]

Hi,

The attached 3 patches add the "-manage-gids" option to the NFS server.
When enabled, for AUTH_SYS RPC requests, the server will use the list of
groups that are assigned to the uid on the server instead of the list of
gids in the RPC request.

This can be used to avoid the limit of 16 groups in the RPC header.

It does require that the NFS server be configured with the same passwd and
group databases as the NFS client(s). It uses getgrouplist(3) on the server
to get the group list, so that must work correctly on the server.
It uses the same cache as the name<->id cache used for NFSv4. As such,
the option is on the nfsuserd command line and it must be running for this
to work for NFSv3 and/or NFSv4.

Josh Paetzel already asked if this option was reasonable and got a positive
response.

Please review and/or test these patches, rick
ps: I tried to put them in reviews.freebsd.org, but failed miserably, so
    I've attached them here.
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