Revert "Ganeti Filter Language: Delete `qlangOpNotEqual`"

This reverts commit 97e7ea3fa918369c9dd6163fa021b8ca5e93891d.

The "!=" operation never worked on the command line, but it did work
in 2.10 via RAPI.

Thus we decided to not remove "!=", but instead try to make it work
as a proper alias of "not (... == ...)".

Signed-off-by: Niklas Hambuechen <niklash@google.com>