config: Added support for specifying include filter patterns.

An optional second argument may now be passed to the "Include" configuration
option. If specified (and if the fnmatch() function was available at build
time), only files matching this pattern will be included. For example, the
following will include all files matching "*.conf" in any subdirectory of
/etc/collectd.d/:

  Include "/etc/collectd.d" "*.conf"

This is useful, e.g. for distributions in order to include a possibly empty
directory in the default configuration including all subdirectories but also
making it possible to ship further documents like README files.
aggregation plugin: Make it possible to set parts of the identifier.
config: Implemented include filter pattern as sub-option in a <Include> block.

This has the advantage of being more easy to read/understand and also being
more flexible. E.g. multiple filters and other options may be specified in the
future.

Thanks to octo for suggesting this!
configfile: Free a string buffer returned by cf_util_get_string().
Merge branch 'sh/config-include'
apcups plugin: Implement the "ReportSeconds" option.
Remove the redundant and little used STATIC_ARRAY_LEN() macro.