[RFC] default to 'nocompatible'

I'm not really that happy with the changes I had to make to the first test in order to get it to work, so maybe someone else has some ideas to make it a bit better.

For the "this shouldn't be deleted" test, the actual output doesn't really make much sense (the line that says it should be deleted is not deleted, and the one that says it should not be modified is modified), so I made it expect the output that makes sense
Let's change this to 

    EXTERN char_u e_unsupportedoption[] INIT(= N_("E519: Option not supported"));

And update the existing line 2620 in option.c to:

    errmsg = e_unsupportedoption;