Implement challenge/response authentication support in client mode,

Implement challenge/response authentication support in client mode,
where credentials are entered from stdin.  This capability is
compiled when ENABLE_CLIENT_CR is defined in syshead.h (enabled
by default).

Challenge/response support was previously implemented for creds
that are queried via the management interface.  In this case,
the challenge message will be returned as a custom
client-reason-text string (see management-notes.txt for more
info) on auth failure.

Also, see the comments in misc.c above get_auth_challenge()
for info on the OpenVPN challenge/response protocol.

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@6568 e7ae566f-a301-0410-adde-c780ea21d3b5