Perform only one ReadControlFile() during startup.

Previously we read the control file in multiple places. But soon the
segment size will be configurable and stored in the control file, and
that needs to be available earlier than it currently is needed.

Instead of adding yet another place where it's read, refactor things
so there's a single processing of the control file during startup (in
EXEC_BACKEND that's every individual backend's startup).

Author: Andres Freund
Discussion: http://postgr.es/m/20170913092828.aozd3gvvmw67gmyc@alap3.anarazel.de

src/backend/postmaster/postmaster.c

@@ -950,6 +950,9 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	CreateDataDirLockFile(true);
 
+	/* read control file (error checking and contains config) */
+	LocalProcessControlFile();
+
 	/*
 	 * Initialize SSL library, if specified.
 	 */
@@ -4805,6 +4808,9 @@ SubPostmasterMain(int argc, char *argv[])
 	/* Read in remaining GUC variables */
 	read_nondefault_variables();
 
+	/* (re-)read control file (contains config) */
+	LocalProcessControlFile();
+
 	/*
 	 * Reload any libraries that were preloaded by the postmaster.  Since we
 	 * exec'd this process, those libraries didn't come along with us; but we