forb: Fix synchronization error in forbrun

forb: Fix synchronization error in forbrun

forbrun starts servers synchronously, i.e. the next server is started
only after the previous one is fully initialized (see
forb_signal_server_ready()). This synchronization was broken because
forb_signal_server_ready() was incorrectly called in
forb_execute_object() called from forb_execution_thread() called from
forb_init(). This caused that the semaphore for the synchronization had
the value of one (instead of zero) when forb_init() finished.
Therefore, the first waiting for the semaphore returned immediately
without actually waiting for the server.

This is now fixed by introduction of default_executor which is declared
in struct forb. This executor is initialized "manually", without
forb_execute_object(), so that the forb_signal_server_ready() is not
called.

In future, we might want use the default executor for executing requests
to other objects, which do not require a separate thread.