Prevent infinite loop in start helper script

The start helper script gets stuck in an infinite loop (which causes
broctl to hang) if it cannot execute the run-bro script.  Added a
check if that script exists (and is executable), and exit with error
if not.
Simplify code that execs commands locally

Removed the captureCmd function (which could cause broctl to hang
due to filling the OS pipe buffer, although this doesn't actually
happen with any of the current uses of this function), and use runLocalCmd
instead.  This allows removal of some code in the popen wrapper function.