Suppress log spam from multiple reports of SIGQUIT shutdown.

When the postmaster sends SIGQUIT to its children, there's no real
need for all the children to log that fact; the postmaster already
made a log entry about it, so adding perhaps dozens or hundreds of
child-process log entries adds nothing of value.  So, let's introduce
a new ereport level to specify "WARNING, but never send to log" and
use that for these messages.

Such a change wouldn't have been desirable before commit 7e784d1dc,
because if someone manually SIGQUIT's a backend, we *do* want to log
that.  But now we can tell the difference between a signal that was
issued by the postmaster and one that was not with reasonable
certainty.

While we're here, also clear error_context_stack before ereport'ing,
to prevent error callbacks from being invoked in the signal-handler
context.  This should reduce the odds of getting hung up while trying
to notify the client.

Per a suggestion from Andres Freund.

Discussion: https://postgr.es/m/20201225230331.hru3u6obyy6j53tk@alap3.anarazel.de

src/include/utils/elog.h

@@ -40,19 +40,19 @@
 #define WARNING		19			/* Warnings.  NOTICE is for expected messages
 								 * like implicit sequence creation by SERIAL.
 								 * WARNING is for unexpected messages. */
-#define ERROR		20			/* user error - abort transaction; return to
+#define WARNING_CLIENT_ONLY	20	/* Warnings to be sent to client as usual, but
+								 * never to the server log. */
+#define ERROR		21			/* user error - abort transaction; return to
 								 * known state */
 /* Save ERROR value in PGERROR so it can be restored when Win32 includes
  * modify it.  We have to use a constant rather than ERROR because macros
  * are expanded only when referenced outside macros.
  */
 #ifdef WIN32
-#define PGERROR		20
+#define PGERROR		21
 #endif
-#define FATAL		21			/* fatal error - abort process */
-#define PANIC		22			/* take down the other backends with me */
-
- /* #define DEBUG DEBUG1 */	/* Backward compatibility with pre-7.3 */
+#define FATAL		22			/* fatal error - abort process */
+#define PANIC		23			/* take down the other backends with me */
 
 
 /* macros for representing SQLSTATE strings compactly */