Log to subfolders of a base path

A different subfolder for each protocol.
Remove irc: from irc message filenames
Add a comment about existence of logging dirs
Update IRC logging to be more idiomatic

Based on a message from kini on #oplss at 2013-07-27 12:40:
   freenode / #oplss / kini  2013-07-27 12:40  (kini!kini@unaffiliated/kini)
       jgross: the standard format is "[timestamp] <username> message", one line per message
   freenode / #oplss / kini  2013-07-27 12:41  (kini!kini@unaffiliated/kini)
       non-message events are usually notated as "* event description", e.g. "* kini (unaffiliated/kini) joined
       #oplss", or whatever
Use g_utf8_casefold and g_utf8_normalize

We define a convenience function compat_casefold in util.c for reuse in
filters.
Add log-to-subdirectories, a compatibility frob

If disabled (the default), then logging of jabber and AIM messages
behaves how it used to.  If enabled, then each protocol gets its own
subdirectory.
Log loopbacks to loopback file in ~/zlog

Not to ~/zlog/loop{sender,recip}.  If compatibility logging is on,
loopbacks get logged to ~/zlog/people/loopback, which is what happened
before this branch.