Add vmail command for changing password when you know the current password

Not 100% sure if this the best way, but the members portal was tied to
*the* mail node, which is not good to begin with, and breaks when
there are multiple mail nodes.

 * Replaces vmailpasswd.c, which is an awful program (passed password on
   the command line revealing it to `ps' and only supports a local
   filesystem userdb).
 * Restricted to users with the priv `vmail' for now, and only used by
   the portal. Not much worth in exposing generally it seems (vmail
   users cannot login to any shell machines, at least at hcoop)
 * Includes helper python program to run crypt() (better than C at
   least...)
 * New function to parse the userdb into a StringMap (a better
   approach is possible, similar to the Vmail.list). Will be used to
   compile the database for Dovecot later.
 * New binary `domtool-portal' to expose replacement vmailpasswd command