Added support for more than one TypesDB file.

Added support for more than one TypesDB file.

The "TypesDB" config option now accepts more than one filename. Each file will
be read in the specified order. If no filename has been given, the default
file will _not_ be read (I doubt this is a useful feature but it's imho the
most reasonable behavior).

This may, for example, be used to specify an additional file containing custom
data-set definitions. See the thread "Thought about exec and types.db" on the
mailing-list ([1]).

[1] http://mailman.verplant.org/pipermail/collectd/2008-January/001450.html

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>