ids.cgi: Rework handling of enabled/disabled sids

ids.cgi: Rework handling of enabled/disabled sids

Now the enabled or disabled sids are stored in a single
hash instead of two arrays, which easily can be modified.

When saving the ruleset, the new read_enabled_disabled_sids() function
will be used to read-in the current (old) saved enabled or disabled sids
and add them to the new hash structure.

After adding or modifiying sids to the hash, the entries will be written
to the corresponding files.

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