WIP authorization token documentation
Wip reauthorize component

FIXUP reauthorize library/tool fix
common: Backport g_unix_fd_add() and friends

Backport the new g_unix_fd_add() and related functions which
define GSource's for unix fds. This prevents us from having to
define a custom GSource in CockpitPipe.

In addition we'd like to use these sources for the reauthorization
work.
WIP make transport an abstract base class
common: Add new CockpitTransport::control signal

This signal is fired when a control message is received by the
transport, and no recv signal handler has processed it. We use it
in the agent, to have multiple handlers for control messages,
but still avoiding parsing and validating them multiple times.

In the case of the web service, since we're mainly forwarding
messages, we don't use this new signal.
WIP track creds per session

FIXUP session creds
WIP Implement reauthorize

FIXUP Implement reauthorize
WIP polkit agent