container: make use of sync pipe for error handling

- init process uses sync pipe to report errors
- init process uses init() to hijack execution
- handoff to init process no longer relies on PID namespacing to work
container: keep log streams separate

- harpoon-container will only ever write to stderr
- user processes will only ever be emitted on stdout
agent: simplify logging setup

- stderr of harpoon-container can be written directly to a log file
- svlogd config is greatly simplified
container: log final state
Merge pull request #41 from soundcloud/harpoon-container-refactor

container: better error handling; simpler logging