Absolute thresholds

I hacked a little to make it accept absolute threshold values. Integers/floats will still be interpreted as percentages, but 500M would be an absolute threshold.

Also, the program will not accept bogus input anymore like this:
$ ./check_mem.pl -f -C -w 99.999 -c 1monkeypoop
Argument "1monkeypoop" isn't numeric in numeric eq (==) at ./check_mem.pl line 318.
WARNING - 26.2% (2145244 kB) free!|TOTAL=8177544KB;;;; USED=6032300KB;81;8095768;; FREE=2145244KB;;;; CACHES=1807308KB;;;;

Hope this will be useful for someone.