More class linting fixes. Fixes gh-1956

- Pass function name to doFunction() as a string literal or false, rather than as
  a token.
- Do not emit possible strict violation warnings when `this` is used in a class method.
  (`this` is ordinarily either an instance or the class itself).
Do not modify global options object

Because a class method body may define named functions, the default
behavior of the `validthis` option should be preserved.