ids-functions.pl: Use If-Modified-Since header to reduce file downloads.

ids-functions.pl: Use If-Modified-Since header to reduce file downloads.

When using the "If-Modified-Since" header, the server can be requested
if a modified version of the file can be served.

In case that is true, the file will be sent and stored by the downloader
function. If the file has not been touched since the last time, the
server will respond with the code "304" (Not modified).

This tells us, that the current stored file is the latest one (still up-to-date)
and we safely can skip the download attempt for this provider.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>