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.