container: better error handling; simpler logging

- harpoon-container handles post-fork errors gracefully
- harpoon-container doesn't require "NEWPID" namespace to function
- log output from harpoon-container is strictly separated from user logs and small enough to be written to an unrotated file, allowing the svlogd config to be greatly simplified

harpoon-container's log file (here, "container.log") is confusing, but will be renamed in subsequent PRs to something like "supervisor.log".

/cc @peterbourgon