system: configurable hostname (device identity)

Add a device hostname settable from the CLI (`hostname <text>`) and the
System Settings page. The value is sanitized on ingest to JSON-safe
printable ASCII (<=23 chars), stored in a shared __xdata buffer, seeded
to "RTLPlayground" at boot, persisted through the startup-config, and
reported in /information.json. It lives in the common header so other
modules can advertise it (LLDP uses it as the System Name TLV).

httpd/page_impl.c

@@ -252,6 +252,12 @@ void send_basic_info(void)
 	byte_to_html(uip_ethaddr.addr[3]); char_to_html(':');
 	byte_to_html(uip_ethaddr.addr[4]); char_to_html(':');
 	byte_to_html(uip_ethaddr.addr[5]);
+	slen += strtox(outbuf + slen, "\",\"hostname\":\"");
+	{
+		__xdata char *hp = hostname;	/* sanitized on ingest, emit verbatim */
+		while (*hp)
+			char_to_html(*hp++);
+	}
 	slen += strtox(outbuf + slen, "\",\"sw_ver\":\"");
 	slen += strtox(outbuf + slen, VERSION_SW);
 	slen += strtox(outbuf + slen, "\",\"build_date\":\"");