vfs: add vfs_shell_snap module

vfs: add vfs_shell_snap module

The shell_snap VFS module plumbs into the snapshot (aka shadow-copy)
management paths used by Samba's File Server Remote VSS Protocol (FSRVP)
server.
The following shell callouts may be configured in smb.conf:

shell_snap: check path command
	- Called when an FSRVP client wishes to check whether a given
	  share supports snapshot create/delete requests.
	- The command is called with a single <share path> argument.
	- The command must return 0 if <share path> is capable of being
	  snapshotted.

shell_snap: create command
	- Called when an FSRVP client wishes to create a snapshot.
	- The command is called with a single <share path> argument.
	- The command must return 0 status if the snapshot was
	  successfully taken.
	- The command must output the path of the newly created snapshot
	  to stdout.

shell_snap: delete command
	- Called when an FSRVP client wishes to delete a snapshot.
	- The command is called with <base share path> and
	  <snapshot share path> arguments.
	- The command must return 0 status if the snapshot was
	  successfully removed.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>