The default value is obviously false everywhere so we just return zero
if nothing is found, instead of undef.

Removing an exception from the otherwise quite systematic syntax.
Closes #111.

rt-show.c: fixed OBSREF_SET after cf_error(), changed message when 'show route export' called on static protocol

The restart timer was racy and didn't allow for immediate restarts
from limits. Now the protocols stores the last restart time and in case
of too frequent autorestarts caused by exceeded limits, the protocol
gets disabled with an error message.

Also now there is a configuration knob for this.

There was a race condition in the test itself,
causing the test reader access data after free.

There were some nasty problems with deferred protocol state updates and
race conditions on BGP startup, shutdown, and also with referencing the
cached states.

Now it looks fixed.

What was PS_DOWN before, is now PS_DOWN and PS_FLUSH.

These may cause runaway pointers.

Tests for MRT are scarce and not automated for now, so it may behave
weirdly in corner cases.

To allow reading of protocol states from other protocols or completely
different routines, we have to export these states to data structures
not requiring to lock the protocol loops.

On one hand, this doesn't give the reader the actual state "right now",
on the other hand, getting that is impossible in a properly
multithreaded environment and you will always get the information with
some (little but noteworthy) delay.

This implementation handles only the basic state information of the
protocols, common for all the protocols. Adding protocol-specific state
information should be done by implementing the protocol hook init_state().

Channel information is stored but not announced, as we don't need the
announcements for now.