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.

rtlplayground.c

@@ -278,6 +278,32 @@ void itoa(uint8_t v)
 }
 
 
+/* Same as itoa(), one decade wider: enough for a port number. Kept separate
+ * rather than widening itoa() itself, because every existing caller passes a
+ * byte and would start paying for 16-bit divisions it does not need. */
+void itoa_short(uint16_t v)
+{
+	uint8_t t = v / 10000;
+	uint8_t print_zeros = t;
+
+	if (print_zeros)
+		write_char('0' + t);
+	t = (v / 1000) % 10;
+	print_zeros |= t;
+	if (print_zeros)
+		write_char('0' + t);
+	t = (v / 100) % 10;
+	print_zeros |= t;
+	if (print_zeros)
+		write_char('0' + t);
+	t = (v / 10) % 10;
+	print_zeros |= t;
+	if (print_zeros)
+		write_char('0' + t);
+	write_char('0' + (v % 10));
+}
+
+
 void print_string(__code char *p)
 {
 	while (*p)