The conversion from @ to @@ and so forth is coded is too many
different places. Factor, a bit.
* src/scan-code.l: Instead of duplicating the logic of obstack_escape,
use it.
It sure is less efficient, but the cost is negligible.
This allows to factor rules that are alike.
And to factor some start-condition clauses.
* tests/input.at (Stray $ or @): New.
* NEWS: Document it.