Fixing pidfile creation bug

If osqueryd was killed and another process was started with osqueryd's
old pid before a new osqueryd could start, osqueryd would encounter a
bug where osqueryd would never start.

This executes an osquery query to the processes table to make sure that
the name of the process is "osqueryd". Of course, you could perhaps
denial of service osqueryd this way, but that would require root
filesystem access (assuming that the last version of osqueryd was
ran as root). Thoughts?
Merge pull request #531 from facebook/pidfile-fix

Fixing pidfile creation bug