Add a minor feature to loghole to support network link trace files.

	* utils/loghole.in, utils/loghole.1: Create a directory for traced
	network links that contains symbolic links to the pcap files.

	* www/tutorial/advanced.html: Fix the link to the loghole doc.

	* xmlrpc/emulabserver.py.in: Add a 'traces' aspect to the
	experiment.info method (used by loghole).