MFC: r291536
Add support for the new "-manage-gids" option for the nfsuserd daemon
to the rc scripts. With these changes, setting nfs_server_managegids="YES"
in /etc/rc.conf will enable this capability.

Relnotes: yes

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Prepare for the removal of set_rcvar() by changing the rcvar=
assignments to the literal values it would have returned.

The concept of set_rcvar() was nice in theory, but the forks
it creates are a drag on the startup process, which is especially
noticeable on slower systems, such as embedded ones.

During the discussion on freebsd-rc@ a preference was expressed for
using ${name}_enable instead of the literal values. However the
code portability concept doesn't really apply since there are so
many other places where the literal name has to be searched for
and replaced. Also, using the literal value is also a tiny bit
faster than dereferencing the variables, and every little bit helps.

Add support for the experimental nfs subsystem to the scripts in
/etc/rc.d. They use the following new rc variables:
nfsv4_server_enable - set to "YES" to run the experimental server
nfsuserd_enable - set to "YES" to run nfsuserd for NFSv4 client and
server
nfsuserd_flags - command line flags for nfsuserd
nfscbd_enable - set to "YES" to run the experimental nfs client's
NFSv4 callback daemon
nfscbd_flags - command line flags for nfscbd

Reviewed by: dougb
Approved by: kib (mentor)