Improve agent robustness and cleanup

Improved the page timeout handler to prevent race conditions and allow more time for posting results.

Improved the getwork/workdone handler to catch I/O errors and recover correctly:  wpt_client callbacks now use the fs-style "(error, value)" pattern.

Added "--killall 1" support to run an agent as a "device" user, e.g.:
    sudo -u deviceX ./wptdriver --killall 1 ...
and `killall` zombie processes between runs.  Cleaned up process_util's get/kill functions.