Don't allow unhandled POSTs to write to the filesystem by default

If there's no registered handler for a POST request, the default behaviour
is to write it to the filesystem. Several million deployed devices appear
to have this behaviour, making it possible to (at least) store arbitrary
data on them. Add a configure option that enables this behaviour, and change
the default to just drop POSTs that aren't directly handled.
void web_server_destroy(void)
{
	if (bWebServerState == WEB_SERVER_ENABLED) {
		membuffer_destroy(&gDocumentRootDir);
		alias_release(&gAliasDoc);

		ithread_mutex_lock(&gWebMutex);
		memset(&gAliasDoc, 0, sizeof(struct xml_alias_t));
		ithread_mutex_unlock(&gWebMutex);

		ithread_mutex_destroy(&gWebMutex);
		bWebServerState = WEB_SERVER_DISABLED;
	}
}

void web_server_destroy(void)
{
	if (bWebServerState == WEB_SERVER_ENABLED) {
		membuffer_destroy(&gDocumentRootDir);
		alias_release(&gAliasDoc);

		ithread_mutex_lock(&gWebMutex);
		memset(&gAliasDoc, 0, sizeof(struct xml_alias_t));
		ithread_mutex_unlock(&gWebMutex);

		ithread_mutex_destroy(&gWebMutex);
		bWebServerState = WEB_SERVER_DISABLED;
	}
}
