Add `timeout` option to `P.spawn` to enforce CPU limits.

Change-Id: Ided3b60d0df794048764c5a05a8b6312f9c447e4
Add `--helper-exec-limit` CLI option to protect against runaway image helpers.

The `rsvg-convert` helper was running for over an hour on a 335K SVG file.

Change-Id: I916cd37ac69e41ffc64430d2f506b7d5208e1145