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.
static UPNP_INLINE int get_alias(
	/*! [in] request file passed in to be compared with. */
	const char *request_file,
	/*! [out] xml alias object which has a file name stored. */
	struct xml_alias_t *alias,
	/*! [out] File information object which will be filled up if the file
	 * comparison succeeds. */
	UpnpFileInfo *info)
{
	int cmp = strcmp(alias->name.buf, request_file);
	if (cmp == 0) {
		UpnpFileInfo_set_FileLength(info, (off_t)alias->doc.length);
		UpnpFileInfo_set_IsDirectory(info, FALSE);
		UpnpFileInfo_set_IsReadable(info, TRUE);
		UpnpFileInfo_set_LastModified(info, alias->last_modified);
	}

	return cmp == 0;
}

static UPNP_INLINE int get_alias(
	/*! [in] request file passed in to be compared with. */
	const char *request_file,
	/*! [out] xml alias object which has a file name stored. */
	struct xml_alias_t *alias,
	/*! [out] File information object which will be filled up if the file
	 * comparison succeeds. */
	UpnpFileInfo *info)
{
	int cmp = strcmp(alias->name.buf, request_file);
	if (cmp == 0) {
		UpnpFileInfo_set_FileLength(info, (off_t)alias->doc.length);
		UpnpFileInfo_set_IsDirectory(info, FALSE);
		UpnpFileInfo_set_IsReadable(info, TRUE);
		UpnpFileInfo_set_LastModified(info, alias->last_modified);
	}

	return cmp == 0;
}
