[toplevel] Update state when `Drop` exception is thrown [#6872]

`Drop` is implemented using exceptions-as-control flow, so the
toplevel state gets corrupted as `do_vernac` will never return when
`Drop` occurs in the input.

The right fix would be to remove `Drop` from the vernacular and make
it a toplevel-only command, but meanwhile we can just patch the state
in the exception handler.

We also need to keep the global state in `Coqloop` as the main
`coqtop` entry point won't be called by `go ()`.

Fixes #6872.

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