Added flag to chose munin subfield

[TinLe]

Not really an issue. My munin graphs have multiple items being tracked, e.g. CPU will have user, system, idle, steal, etc. and the plugin returns all of them, confusing Nagios.

I've added a flag to pick submodule. So something like this:

original:

check_munin_rrd.pl -H foo -d foo -M cpu -w 10 -c 20
CRITICAL - idle value 185.5, is above critical treshold 20

new:

check_munin_rrd.pl -H foo -d foo -M cpu -S system -w 10 -c 20
system OK - system: 1.0

I can submit my patch if anyone is interested.