core: add function and testcase to parse vpnc stdout/stderr

We'll use this to run vpnc in interactive mode and handle server
messages and input prompts during the connection, instead of all
before connecting.
core: parse vpnc stdin/stdout and request secrets asynchronously

Parse vpnc's output to recognize server messages and prompts for
passwords and other information.  This requires vpnc SVN 550 or
later.
auth-dialog: parse hints and display server messages

The auth dialog already has all the code to do this, so just flip
the switch.
merge: support interactive VPN secrets requests (bgo #703277)