Clean up some inconsistencies with GUC declarations

This is similar to 7d25958, and this commit takes care of all the
remaining inconsistencies between the initial value used in the C
variable associated to a GUC and its default value stored in the GUC
tables (as of pg_settings.boot_val).

Some of the initial values of the GUCs updated rely on a compile-time
default.  These are refactored so as the GUC table and its C declaration
use the same values.  This makes everything consistent with other
places, backend_flush_after, bgwriter_flush_after, port,
checkpoint_flush_after doing so already, for example.

Extracted from a larger patch by Peter Smith.  The spots updated in the
modules are from me.

Author: Peter Smith, Michael Paquier
Reviewed-by: Nathan Bossart, Tom Lane, Justin Pryzby
Discussion: https://postgr.es/m/CAHut+PtHE0XSfjjRQ6D4v7+dqzCw=d+1a64ujra4EX8aoc_Z+w@mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -196,7 +196,7 @@ BackgroundWorker *MyBgworkerEntry = NULL;
 
 
 /* The socket number we are listening for connections on */
-int			PostPortNumber;
+int			PostPortNumber = DEF_PGPORT;
 
 /* The directory names for Unix socket(s) */
 char	   *Unix_socket_directories;