check_fping error handling enhanced, generates UNKNOWN on
fping parameter error (i.e. wrong interface, missing perms ..)
Merge branch 'fping' of https://github.com/lairsdragon/nagios-plugins

* 'fping' of https://github.com/lairsdragon/nagios-plugins:
  check_fping error handling enhanced, generates UNKNOWN on fping parameter error (i.e. wrong interface, missing perms ..)

Closes #61.