psql: initialize comment-begin setting to a useful value by default.

Readline's meta-# command is supposed to insert a comment marker
at the start of the current line.  However, the default marker is
"#" which is entirely unhelpful for SQL.  Set it to "-- " instead.
(This setting can still be overridden in one's ~/.inputrc file,
so this change won't affect people who have already taken steps
to make the command useful.)

Discussion: https://postgr.es/m/CAJcOf-cAdMVr7azeYR7nWKsNp7qhORzc84rV6d7m7knG5Hrtsw@mail.gmail.com

src/bin/psql/input.c

@@ -353,8 +353,13 @@ initializeInput(int flags)
 
 		useReadline = true;
 
-		/* these two things must be done in this order: */
+		/* set appropriate values for Readline's global variables */
 		initialize_readline();
+
+		/* set comment-begin to a useful value for SQL */
+		(void) rl_variable_bind("comment-begin", "-- ");
+
+		/* this reads ~/.inputrc, so do it after rl_variable_bind */
 		rl_initialize();
 
 		useHistory = true;