to restart camli properly
This change adds the functionality to find the path to
the currently running executable, so that camlistore
can be restarted without à priori knowledge of cwd.
It should work for linux, netBSD and freeBSD, and it
might work for openBSD.
The naive (os.Args[0]) approach is still used with
darwin.
Reference: https://codereview.appspot.com/6736069/
Change-Id: Icc9fc2b4e1b3542cfbe6c457c1fefbe762eff2f1
Brad Fitzpatrick