Add Log::rotation_format_func and Log::default_rotation_dir options

These may be redefined to customize log rotation path prefixes,
including use of a directory.  File extensions are still up to
individual log writers to add themselves during the actual rotation.

These new also allow for some simplication to the default
ASCII postprocessor function: it eliminates the need for it doing an
extra/awkward rename() operation that only changes the timestamp format.

This also teaches the supervisor framework to use these new options
to rotate ascii logs into a log-queue/ directory with a specific
file name format (intended for an external archiver process to
monitor separately).

src/zeek-setup.cc

@@ -643,6 +643,7 @@ zeek::detail::SetupResult zeek::detail::setup(int argc, char** argv,
 		exit(1);
 
 	iosource_mgr->InitPostScript();
+	log_mgr->InitPostScript();
 	plugin_mgr->InitPostScript();
 	zeekygen_mgr->InitPostScript();
 	broker_mgr->InitPostScript();