Factor out InitControlFile() from BootStrapXLOG()
Right now this only makes BootStrapXLOG() a bit more manageable, but
in the future there may be external callers.
Discussion: https://www.postgresql.org/message-id/
e8f86ba5-48f1-a80a-7f1d-
b76bcb9c5c47@2ndquadrant.com