SMB3: parsing for new snapshot timestamp mount parm

SMB3: parsing for new snapshot timestamp mount parm

BugLink: http://bugs.launchpad.net/bugs/1670508
New mount option "snapshot=<time>" to allow mounting an earlier
version of the remote volume (if such a snapshot exists on
the server).

Note that eventually specifying a snapshot time of 1 will allow
the user to mount the oldest snapshot. A subsequent patch
add the processing for that and another for actually specifying
the "time warp" create context on SMB2/SMB3 open.

Check to make sure SMB2 negotiated, and ensure that
we use a different tcon if mount same share twice
but with different snaphshot times

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
(cherry picked from commit 8b217fe7fcadd162944a88b14990b9723c27419f)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>