Add -manage-gids option to the NFS server

[rmacklem]

Summary

Add an option to the NFS similar to the "--manage-gids" option for rpc.mountd in Linux.
This option is enabled via the "-manage-gids" command line option to the nfsuserd daemon.
When enabled, AUTH_SYS NFS requests use the set of groups for the user for the uid in the
request on the server instead of the list of groups in the RPC request header.

This is useful, since the list of groups in the RPC request header is limited to 16.
It does require that the server be configured with the same passwd and group
database as the NFS client(s) are using.

Test Plan

I have done limited testing and jpaetzel@ will be doing testing as well.
My limited testing consisted of creating a user in more than 16 groups and
trying to access a directory with mode 0770 that was owned by another user
and a group not in the first 16 for the user. It fails without "-manage-gids",
but access works when "-manage-gids" is enabled.

[rmacklem]

Here is the diff for nfsuserd. The previous one is the kernel.

[rmacklem]

This patch is for the rc scripts. The previous one is the kernel changes.

[jpaetzel]

We've been using this in FreeNAS for a while, looks good.

[rmacklem]

The commit has been done to head and MFC'd and no reports of problems w.r.t. have been received.