agent: Implement our own dummy polkit reauthorization agent

cockpit-agent now tells polkit that we're ready to handle
reauthorization requests. Although we don't handle any prompting
in our polkit agent. We rely on pam_reauthorize.so to send us
reauthorize requests via it's back channel instead.
web: Implement a basic 'Drop extra privileges' menu option

This tells cockpit-ws not to reauthorize polkit requests any longer.
May need some tweaking in the future, and probably needs to be
timeout driven in addition to this.

Includes the concept of poisoning CockpitCreds objects, so that while
still valid objects, the actual credentials (like password) are no
longer usable.
reauthorize: A much simpler reauthorize mechanism

 * We only support a polkit-agent now and the polkit agent reauthorizes
   in its helper using reauthorize_perform().
 * The pam_reauthorize.so module is still around, but it's only use is
   to derive the secret from the login password and put in the keyring.
 * No more listening/connecting to a reauthorization agent.
 * sudo is not covered by this, but the plan is to have a separate
   PAM module which reauthorizes sudo requests against polkit. All
   requests go through polkit.

To enable privileged tests, do:

 $ make enable-root-tests

Fixes #483
agent: Fix textstream to return exit-status and exit-signal

These codes weren't being set properly due to an invalid check.
Integration tests, distcheck, for reauthorize code

This adds an 'internal' page for testing these sorts of things.
In the long term we don't have to distribute this page as one of
our default modules.