ids.cgi: Add some more sanity checks when adding a new provider.

ids.cgi: Add some more sanity checks when adding a new provider.

* Check if the system is online.
* Check if enough free disk space is available.
* Abort whith an error message if the ruleset could not be
  downloaded.

In error case the provider now will be removed again from the file which
keeps the configured providers. Sadly it needs to be added first because
otherwise the downloader could not read the required values from it.....

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