compare modifiers for equality, not subset (+test)

compare modifiers for equality, not subset (+test)

Subset comparison was introduced with the rather large commit
bf3cd41b5ddf1e757515ab5fbf811be56e5f69cc, but I now think we should use
equality.

In other words, the following key binding:

    bindsym Mod4+x nop Mod4+x

previously would have been triggered when pressing Mod3+Mod4+x.

Strictly speaking, this is a change of behavior, but it breaks none of our
tests, and using equality instead of subset comparison enables more use-cases.

fixes #2002