When a check times out using alarm, any processes launched using cache_command
or backticks would still be running, and leave a pipe open, even after the
process exits. This change keeps track of any child processes, and will kill
them off and clean up the pipe if a check timeout occurs. Modules that use
cache_command do not need any modification, but modules that use backticks
should change to using Resmon::ExtComm::run_cmd, which is a drop in
replacement. (use run_cmd("command") instead of `command`).

git-svn-id: https://labs.omniti.com/resmon/trunk@240 8c0face9-b7db-6ec6-c4b3-d5f7145c7d55