cifs: use standard token parser for mount options

Use the standard token parser instead of the long if condition to parse
cifs mount options.

This was first proposed by Scott Lovenberg
http://lists.samba.org/archive/linux-cifs-client/2010-May/006079.html

Mount options have been grouped together in terms of their input types.
Aliases for username, password, domain and credentials have been added.
The password parser has been modified to make it easier to read.

Since the patch was first proposed, the following bugs have been fixed
1) Allow blank 'pass' option to be passed by the cifs mount helper when
using sec=none.
2) Do not explicitly set vol->nullauth to 0. This causes a problem
when using sec=none while also using a username.
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>