Fix indentation regressions.

The regressions were caused by extending go-dangling-operators-regexp
with non-operators, so it could be used for line-continuation
detection. The fix is to define and use an independent
go-line-continuation-regexp, and to restore
go-dangling-operators-regexp to its previous value. A new
go-previous-line-has-line-continuation-p has been defined, and a
go-line-continuation cache has been added.