frontend: Let LIST not include details.

Now that we have FETCH and LOOKUP, list should only provide basic information
(name and last_update).
store: Calculate and store each objects update interval.

The interval is calculated as the moving average of the incoming update
intervals. This should also work fine for out-of-order updates or updates from
multiple sources.
store: Fixed a conversion warning reported by GCC.
core/time: Added sdb_strfinterval().

This function formats a time interval using the format 1Y2M3D4h5m6.7s.
store: Include update interval in the JSON serialization.
store_test: Added a test for interval calculation.
collectd::unixsock backend: Simplified host updates.

Also, don't do duplicate updates in a single iteration. This caused bad
startup effects for the interval calculation.

Later, we should make the core more robust in that respect, I guess.
store: Moved matcher types to store-private.h.
store: Added sdb_store_matcher_parse_cmp().

This function parses simple matcher compare expressions (<obj_type>.<attr>
<op> <value>).
frontend: Don't ignore single characters in the scanner.

This would lead to those characters be silently ignored at any place in the
input stream.
frontend: Accept compare matchers as expressions.

… replacing the current placeholder.

For now, this is specific to '=' (equal) and '=~' (regex) comparison.