Allow 0 as a threshold value for -w and -c. (reopen)

I want to add a check for negative values for a metric but the "0" on the cmdline is interpreted as "no value specified".

sample:

$ ./check_tsd -m foo.bar -x lt -w 0
Usage: check_tsd [options]

check_tsd: error: You must specify at least a warning threshold (-w) or a critical threshold (-c).

(resubmitting https://github.com/OpenTSDB/opentsdb/pull/123 closed pull request)