Added "auth-token" client directive, which is intended to be

Added "auth-token" client directive, which is intended to be
pushed by server, and that is used to offer a temporary session
token to clients that can be used in place of a password on
subsequent credential challenges.

This accomplishes the security benefit of preventing caching
of the real password while offering most of the advantages
of password caching, i.e. not forcing the user to re-enter
credentials for every TLS renegotiation or network hiccup.

auth-token does two things:

1. if password caching is enabled, the token replaces the
   previous password, and

2. if the management interface is active, the token is output
   to it:

     >PASSWORD:Auth-Token:<token>

Also made a minor change to HALT/RESTART processing when password
caching is enabled.  When client receives a HALT or RESTART message,
and if the message text contains a flags block (i.e. [FFF]:message),
if flag 'P' (preserve auth) is present in flags, don't purge the Auth
password.  Otherwise do purge the Auth password.

Version 2.1.3o

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@7088 e7ae566f-a301-0410-adde-c780ea21d3b5