This adds a new routine to src/common/ which is compatible with both the
frontend and backend code, able to update the control file's contents.
This is now getting used only by pg_rewind, but some upcoming patches
which add more control on checksums for offline instances will make use
of it.  This could also get used more by the backend as xlog.c has its
own flavor of the same logic with some wait events and an additional
flush phase before closing the opened file descriptor, but this is let
as separate work.

Author: Michael Banck, Michael Paquier
Reviewed-by: Fabien Coelho, Sergei Kornilov
Discussion: https://postgr.es/m/20181221201616.GD4974@nighthawk.caipicrew.dd-dns.de