fix "bpipe run" leaking a tail process each time on OSX minor refactoring
fix "bpipe run" leaking a tail process each time on OSX
minor refactoring

tucano/bpipe,tucano/bpipe,tucano/bpipe,vivovip/bpipe,sb43/bpipe,sb43/bpipe,vivovip/bpipe,vivovip/bpipe,sb43/bpipe