support pre-md5summed passwords in 6in4 scripts

support pre-md5summed passwords in 6in4 scripts

The he.net ipv6 tunnels require that a user supply the md5sum of his password
when updating the ipv4 tunnel endpoint. the 6in4 package supports this well,
but requires that a user store his password in plaintext in uci. this patch
tries to detect whether the password in uci is already an md5sum
(32 characters containing only [a-z0-9]), so users can avoid storing plaintext
passwords.

Signed-off-by: Steven Brudenell <steven.brudenell@gmail.com>
SVN-Revision: 22889