Allow --dbus-user=none on the command line for profiles with dbus-user filter

[rusty-snake]

I just tried to start firefox with --private and --dbus-user=none and it failed because firefox.profile sets dbus-user filter and firejail does not allow relaxing the policy. Error: Cannot relax dbus-user policy, it is already set to block. IMHO the must common case is that a user wants to deny dbus access per run or default and add dbus-user none to xxx.local or the command line and shouldn't need to also add ignore dbus-user filter.

[kris7t]

The idea was to prevent setting a weaker DBus policy without explicitly authorizing the weakening with --ignore. But it seems I was a bit overzealous, as this effectively prevents strengthening the policy (if the (--dbus-user= options are not in the right order) also.

I think it would be safe to downgrade this to a warning, and keep the stricter policy of the two specified. I'll add this to #3406.