Agent plugin fix for Debian wheezy

AWK substr function says that "The first character of a string is character number one." Therefore zero should not be used as second argument.

Tested on AWK version mawk 1.3.3 Nov 1996.
Fix for clustered services

Check returns UNKNOWN, as the BIRD is setup to run on master-backup two node server cluster. For monitoring of clustered services it is required, that check_mk_agent on backup node do not return any data if service (BIRD) is not running.
Section <<<bird>>> is sent to output only if birdc is able to contact the daemon.
Merge pull request #2 from EqO/patch-1

Agent plugin fix for Debian wheezy