(request) implement an UNSET ALL that works like UNLOCK ALL

[Dunbaratu]

It wold be handy if there was an UNSET ALL that cleared the value of every variable.

At the moment I'd like it as a workaround to the loading from on-rails problem. (If you have no variable context being kept alive in the persistence file, then the bug does not occur, so make sure to UNSET ALL just before going on-rails).

But I can see other cases where it would be handy - like to test a script from the beginning again and making sure it starts from a clean slate.

[Dunbaratu]

Alternatively, if the act of rebooting the machine caused the variables to clear, that would work too. (It doesn't, by the way, and I don't know if that's a bug or intentional. If you "set x to 5." and then "reboot." and then after rebooting do "print x." it still says "5".

[a1270]

Seems system variables and user-defined ones are stored in the same list. Have to think about how to deal with that.