Start the server if another user has a PID matching our stale pidfile.

If unicorn doesn't get terminated cleanly (for example if the machine
has its power interrupted) and the pid in the pidfile gets used by
another process, the current unicorn code will exit and not start a
server. This tiny patch fixes that behaviour.

Acked-by: Eric Wong <normalperson@yhbt.net>

lib/unicorn/http_server.rb

@@ -656,7 +656,7 @@ def valid_pid?(path)
     wpid <= 0 and return
     Process.kill(0, wpid)
     wpid
-    rescue Errno::ESRCH, Errno::ENOENT
+    rescue Errno::ESRCH, Errno::ENOENT, Errno::EPERM
       # don't unlink stale pid files, racy without non-portable locking...
   end