Allow SSL configuration to be updated at SIGHUP.

It is no longer necessary to restart the server to enable, disable,
or reconfigure SSL.  Instead, we just create a new SSL_CTX struct
(by re-reading all relevant files) whenever we get SIGHUP.  Testing
shows that this is fast enough that it shouldn't be a problem.

In conjunction with that, downgrade the logic that complains about
pg_hba.conf "hostssl" lines when SSL isn't active: now that's just
a warning condition not an error.

An issue that still needs to be addressed is what shall we do with
passphrase-protected server keys?  As this stands, the server would
demand the passphrase again on every SIGHUP, which is certainly
impractical.  But the case was only barely supported before, so that
does not seem a sufficient reason to hold up committing this patch.

Andreas Karlsson, reviewed by Michael Banck and Michael Paquier

Discussion: https://postgr.es/m/556A6E8A.9030400@proxel.se

src/backend/libpq/be-secure.c

@@ -63,16 +63,31 @@ bool		SSLPreferServerCiphers;
 /* ------------------------------------------------------------ */
 
 /*
- *	Initialize global context
+ *	Initialize global context.
+ *
+ * If failOnError is true, report any errors as FATAL (so we don't return).
+ * Otherwise, log errors at LOG level and return -1 to indicate trouble.
+ * Returns 0 if OK.
  */
 int
-secure_initialize(void)
+secure_initialize(bool failOnError)
 {
 #ifdef USE_SSL
-	be_tls_init();
-#endif
-
+	return be_tls_init(failOnError);
+#else
 	return 0;
+#endif
+}
+
+/*
+ *	Destroy global context, if any.
+ */
+void
+secure_destroy(void)
+{
+#ifdef USE_SSL
+	be_tls_destroy();
+#endif
 }
 
 /*