Add custom message for threshold and missings.

Add custom message for threshold and missings.

Hi everybuddy

Add two new options in thresholds, the Message which can define a custom
message for thresholds and MissingMessage, which define a custom message for
missing interesting values related with that threshold.

This work is heavly based on previously custom message patch publishded by
Taizo ITO on Mar-2010 [1]. The code was modify to add more patterns and keep
compatibility with target_notification syntax. Also now we have
a MissingMessage too.

The pattern matching code was moved to a new function in utils_threshold.c
(ut_build_message) which create the message string.

New patterns:

	%{ds:<name>} returns values for ds called "name", as target_notification
	does.
	%{value} return the offending value for the threshold.
	%{data_source} return the datasource name for offending value
	%{hysteresis} return the hysteresis parameter value
	%{hits} return the hits parameter value

[1] http://www.mail-archive.com/collectd@verplant.org/msg00569.html

Thanks again to Taizo for the previously work :)

Enjoy!