perl plugin, Collectd.pm: Added support for filter chains.

perl plugin, Collectd.pm: Added support for filter chains.

fc_register() may now be used to register matches and targets with collectd.
The function takes three arguments: type, name, proc. 'type' has to be any of
FC_MATCH or FC_TARGET, 'name' specifies the name of the target / match and
'proc' is a hash of callbacks (specified by name just like any other
callbacks). There are three types of callbacks: create, destroy and match /
invoke - the former two being optional.

The match / invoke callback has to return any of the following statuses:
FC_MATCH_NO_MATCH, FC_MATCH_MATCHES, FC_TARGET_CONTINUE, FC_TARGET_STOP or
FC_TARGET_RETURN. If none of those constants are used, the behavior is
undefined.