nfs: Support directory level exports

nfsx has followed traditional approach of exporting whole volumes
as NFS exports. The Platform requires and some users have approached us for
introducing exports of only specific directories instead of full Gluster
volumes. This commit introduces this support through two nfsx options:

Option 1:

=========
option nfs3.<volume-name>.export-dir <subdir1-in-vol>,<subdir2-in-vol>,..<subdirN-in-vol>

export-dir will allow the export of a particular dir as a single export
by itself. For eg.:

volume posix
        type storage/posix
        option directory /export/
end-volume

volume posix-ac
        type features/access-control
        subvolumes posix
end-volume

volume nfs
        type nfs/server
        subvolumes posix-ac
        option rpc-auth.addr.allow *
        option nfs3.posix-ac.export-dir /homes/shehjart
end-volume

A comma separate list of sub-directories will set up those dirs as separated exports.
At the nfs client, the mount command will be:

$ mount <nfsserver>:/posix-ac/homes/shehjart /mnt

Option 2:
=========
option nfs3.<volume-name>.export-volumes <on|off>

There can be situations where users only want the directory level
exports and require that volume exports be completely disabled. The
above option allows us to do this. By default, volume exports are
enabled. From the earlier example, replacing <volume-name> with
posix-ac, will disable mounting of the posix-ac volume as a whole.

Signed-off-by: Shehjar Tikoo <shehjart@gluster.com>
Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 989 (Support directory exports in nfsx)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=989

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