always write PID file early for compatibility

This reduces the window for a non-existent PID for folks who monitor
PIDs (not a great idea anyways).  Unfortunately, this change also brings
us back to the case where having a PID later (for other process monitors)
is beneficial but more unicorn releases exist where we write the PID
early.

Thanks to Jimmy Soho for reporting this issue.
ref: <CAHStS5gFYcPBDxkVizAHrOeDKAkjT69kruFdgaY0CbB+vLbK8Q@mail.gmail.com>

This partially reverts 7d6ac0c17eb29a00a5b74099dbb3d4d015999f27

Folks: please monitor your app with HTTP requests rather than checking
processes, a stuck/wedged Ruby VM is still a running one.