syslog: make the destination port configurable

The port was frozen at 514 in the call that opens the connection, so a
collector listening anywhere else was simply out of reach, and plenty do,
because 514 is privileged and containerised collectors usually get a high
port instead. Working around it meant putting a proxy in front, which then
rewrites the source address and hides which device actually sent the line.

Add `syslog port <n>`. The default stays 514, so nothing changes for an
existing configuration. Setting it tears the connection down and builds a
new one, the same dance the address already does, because uip_udp_new()
freezes the port into the connection.

The value is parsed here rather than through atoi_short(), which reports
"no digits" but not overflow: 65540 would quietly wrap to 4 and we would
open a connection to a port nobody asked for. Checking before each multiply
keeps it in 16-bit arithmetic. Zero then covers all three ways to be wrong -
no digits, past 65535, or an explicit "0", which is not a port either.

Whitelist the new command in config.js as well, next to `syslog ip`, or the
line is dropped without a word when the configuration is saved and the
setting quietly fails to survive a reboot. The dedup entry has to precede
the general `^syslog\b` one, otherwise it would deduplicate against
`syslog on`.

itoa_short() is new: itoa() only handles a byte and print_short() prints
hex, so there was no way to show a port as a decimal number. Kept separate
from itoa() rather than widening it, because every existing caller passes a
byte and would start paying for 16-bit divisions it does not need.

The scratch variables for the parser live in xdata. Locals there would take
internal RAM, and this image has none left: data ends at 0x77, the overlay
occupies 0x78-0x7f and the stack starts at 0x80. Measured: no module's DSEG
grew.

syslog.c

@@ -27,20 +27,22 @@ void syslog_init(void) __banked
 	state.readptr = 0;
 	state.line_available = 0;
 	state.server_ip[0] = 0; state.server_ip[1] = 0; state.server_ip[2] = 0; state.server_ip[3] = 0;// Default to 0.0.0.0
+	state.server_port = SYSLOG_PORT_DEFAULT;
 }
 
 void syslog_start(void) __banked
 {
 	if (state.syslog_conn == 0) {
 		uip_ipaddr(server_ip, state.server_ip[0], state.server_ip[1], state.server_ip[2], state.server_ip[3]);
-		state.syslog_conn = uip_udp_new(&server_ip, HTONS(514));
+		state.syslog_conn = uip_udp_new(&server_ip, HTONS(state.server_port));
 		if (state.syslog_conn == 0) {
 			print_string_newline_no_syslog("Failed to create a new UDP client");
 			return;
 		}
 		print_string_newline_no_syslog("Started syslog to IP ");
 		itoa(state.server_ip[0]); write_char('.'); itoa(state.server_ip[1]); write_char('.');
-		itoa(state.server_ip[2]); write_char('.'); itoa(state.server_ip[3]); write_char('\n');
+		itoa(state.server_ip[2]); write_char('.'); itoa(state.server_ip[3]);
+		write_char(':'); itoa_short(state.server_port); write_char('\n');
 		state.enabled = 1;
 	}
 	else {