cifs: add "mfsymlinks" mount option

This is the start for an implementation of "Minshall+French Symlinks"
(see http://wiki.samba.org/index.php/UNIX_Extensions#Minshall.2BFrench_symlinks

).

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>