vfs_shadow_copy2: add snapsharepath parameter

This new parameter defines how to get from the snapshot's
root directory to the share's root directory. It is an
alternative to the "basedir" parameter, but functionally
is a superset of basedir.

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>
