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.

html/config.js

@@ -7,6 +7,7 @@ const conf_cmds = [
   /^netmask\s+(\d{1,3}\.){3}\d{1,3}$/,
   /^syslog\s+(on|off)$/,
   /^syslog\s+ip\s+(\d{1,3}\.){3}\d{1,3}$/,
+  /^syslog\s+port\s+\d{1,5}$/,
   /^passwd\s+\S+$/,
   /^vlan\s+\d{1,4}\s+d$/,
   /^vlan\s+\d{1,4}\s+mgmt$/,
@@ -32,6 +33,7 @@ const conf_overwrite = [
   /^gw\b/,
   /^netmask\b/,
   /^syslog\s+ip\b/,
+  /^syslog\s+port\b/,
   /^syslog\b/,
   /^passwd\b/,
   /^vlan\s+\d{1,4}\s+mgmt$/,