php-collection: add basic support for meta-selections

Add support for new meta selections allowing addition of graphs
with wildcard behavior.
This adds support for @all selection which matches any values
for the given identifier part.
In addition those types for which meta graphs exist now also list the
individual type instances in addition to the meta graph key for separate
graphing.

In order to support such new groups the lookup code has been refactored
to use a single scanning function which recursively traveses the
collectd RRD output directory for hosts, plugins, types and passing the
discovered data to callback functions for use. The callbacks returns
true to indicate traversal should continue on to next depth level and
false to tell it to continue with next element.
e.g. true on a host means it should look for plugins for given host,
false to continue with next host.

Signed-off-by: Florian Forster <octo@huhu.verplant.org>