FreeBSD initscript improvements

Fixed a couple of issues with the FreeBSD rc.d-unicorn.erb script.

1. Improved the command interpreter detection to work with `/usr/bin/env` shebangs.
2. Fixed "bundle exec unicorn" PID/process detection. Fixes status,stop,restart,reload,reopenlogs commands.

As part of this I've also reformatted the script so that the included functions match the _find_processes() patch.

The commit message on d2e9cd3 explains in more depth the "bundle exec unicorn" issue.