Include output on success if only one container is being tested
Add 'prestart' command support.

This adds a 'prestart' command. Currently only SYSV supports this.

Ideal semantics:
- if prestart fails, start will not run
- if prestart fails, restart will not run
- any other conditions are not affected by prestart.

Use cases:
- config checks
- dependency checks
- log a "I am starting!" message somewhere