frontend: Added support for single quotes in string constants.

Just like in SQL, single quotes may be included in a string by specifying two
adjacent single quotes.
sysdbql(7): Documented supported data types.
doc/: Improved documentation around the rrdtool time-series fetcher.
timeseries::rrdtool: Added support for RRDCacheD.

After enabling RRDCacheD support, the plugin supports an additional
time-series fetcher callback which may be used to flush outstanding updates to
an RRD file before fetching data from it.
collectd::unixsock: Support 'rrdcached' metrics as well.

It's the same as 'rrdtool' :-)
configure: Make plugin related options more consistent.

That is, use the full plugin names (except "backend" prefixes) for the
--enable-* options.