Added the ability for the server to provide a custom reason string

Added the ability for the server to provide a custom reason string
when an AUTH_FAILED message is returned to the client.  This
string can be set by the server-side managment interface and read
by the client-side management interface.

For more info, see management/management-notes.txt, and look for
references to "client-reason-text".

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@5012 e7ae566f-a301-0410-adde-c780ea21d3b5