nfs/nlm: procedures for PC clients

*  This change introduces four NLMv4 procedures:

   NM_LOCK, SHARE, UNSHARE and FREE_ALL.

   These are used by PC clients (windows/dos) to control access
   to files.

   1. NM_LOCK: this lock is not monitored by statd.

   2. SHARE: A share reservation is a lock on the whole file
      that is taken whenever a file is opened on windows clients.
      This has ACCESS (N, R, W, RW) and DENY MODE (N, R, W, RW).
        ACCESS:    mode of access requested by the client;
        DENY MODE: what the requesting client wants to
                   deny other clients.

   3. UNSHARE: remove a share reservation obtained by SHARE.
      Called while closing a file.

   4. FREE_ALL: remove all share reservations and locks,
      both monitored and unmonitored, of the calling client.

*  lock and nm_lock use a common function with only
   a flag conveying whether or not to monitor a lock.

*  NOTES:
   1. SHARE reservations are not STACK_WIND'd to subsequent xlators.
      These are maintained in-memory in the nfs xlator.

   2. Consequently, for SHARE reservations to work effectively,
      all PC clients  *must* mount from the same gNfs server.
      Not doing so will result in different servers maintaining
      separate SHARE reservations which will not be enforced
      for obvious reasons.

Change-Id: Id4f22670a94ed58691a6a7f4c80aa8c11421a277
BUG: 800287
Signed-off-by: Rajesh Amaravathi <rajesh@redhat.com>
Reviewed-on: http://review.gluster.com/3212
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishna Srinivas <krishna@gluster.com>

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