Fix upper limit of superuser_reserved_connections, add limit for wal_senders

Should be limited to the maximum number of connections excluding
autovacuum workers, not including.

Add similar check for max_wal_senders, which should never be higher than
max_connections.

doc/src/sgml/config.sgml

@@ -2087,12 +2087,16 @@ SET ENABLE_SEQSCAN TO OFF;
        </indexterm>
        <listitem>
        <para>
-        Specifies the maximum number of concurrent connections from standby
-        servers or streaming base backup clients (i.e., the maximum number of
-        simultaneously running WAL sender
-        processes). The default is zero. This parameter can only be set at
-        server start. <varname>wal_level</> must be set to <literal>archive</>
-        or <literal>hot_standby</> to allow connections from standby servers.
+        Specifies the maximum number of concurrent connections from
+        standby servers or streaming base backup clients (i.e., the
+        maximum number of simultaneously running WAL sender
+        processes). The default is zero, meaning replication is
+        disabled. WAL sender processes count towards the total number
+        of connections, so the parameter cannot be set higher than
+        <xref linkend="guc-max-connections">.  This parameter can only
+        be set at server start. <varname>wal_level</> must be set
+        to <literal>archive</> or <literal>hot_standby</> to allow
+        connections from standby servers.
        </para>
        </listitem>
       </varlistentry>

src/backend/postmaster/postmaster.c

@@ -763,11 +763,16 @@ PostmasterMain(int argc, char *argv[])
 	/*
 	 * Check for invalid combinations of GUC settings.
 	 */
-	if (ReservedBackends >= MaxBackends)
+	if (ReservedBackends >= MaxConnections)
 	{
 		write_stderr("%s: superuser_reserved_connections must be less than max_connections\n", progname);
 		ExitPostmaster(1);
 	}
+	if (max_wal_senders >= MaxConnections)
+	{
+		write_stderr("%s: max_wal_senders must be less than max_connections\n", progname);
+		ExitPostmaster(1);
+	}
 	if (XLogArchiveMode && wal_level == WAL_LEVEL_MINIMAL)
 		ereport(ERROR,
 				(errmsg("WAL archival (archive_mode=on) requires wal_level \"archive\" or \"hot_standby\"")));