travis: --disable-aikgen in "all" tests

aikgen has a hard dependency on TrouSerS, which we currently don't have in the
travis build.
utils: Provide a CALLBACK macro, similar to METHOD, but for void* callbacks

Using the same mechanism as the METHOD macro, the CALLBACK macro defines
a hybrid function signature. It strictly uses a weak void* for the first
function parameter, in contrast to the dynamic METHOD object "this" type.
processor: Flush pending jobs during cancel(), not destroy

During shutdown, cancel queued jobs earlier to avoid having cleanup functions
accessing infrastructure not available anymore, for example watcher.
watcher: Avoid queueing multiple watcher callbacks at the same time

While we don't add FDs with an active callback to the watched FDSET, we still
can get notifications for callbacks active due the asynchronous processing
of the same.

To avoid queue multiple callbacks, we check for queued callbacks before
activating new ones.
watcher: Don't wait for running callback once watcher thread cancelled

During shutdown, waiting for callbacks might never complete, as queued
callbacks might not get executed under certain conditions. Not the clean fix,
but works good enough for now.

Seen on Windows in vici tests.
mem-cred: Replace existing equal private keys during add_key()
mem-cred: Replace existing equal shared keys during add_shared()
vici: Add a plugin stub for the "Versatile IKE Control Interface" plugin
vici: Add a test runner for plugin
vici: Add a fully asynchronous IPC socket segmenting messages on/from stream
vici: Provide a command dispatcher handling request and event registration
vici: Add a libvici low-level client library
vici: Add a query class, currently implementing a list-sas command
vici: Invoke dispatcher outside of connection log, allowing events from commands
vici: Raise events with an optional identifier for specific connections
vici: Add some convenience getters on message
vici: Add convenience value/string/integer getter to libvici
vici: Move dumping to core message class, use it in libvici
vici: Support thread cancellation in command callbacks
vici: Add a control backend, currently to initiate connections by name