Shave a few instructions from child-process startup sequence

https://commitfest.postgresql.org/action/patch_view?id=1277
http://archives.postgresql.org/message-id/CABwTF4Uc+MpK4Bf8RtCNN5wfKBWBWd5x2xr_iZw3SR_etMi9FA@mail.gmail.com