Compiler: Added support for ::this_program.

This syntax refers to the previous definition of the current class
in its parent, and is typically used with inherit like:

  inherit Foo;

  // Override the Bar inherited from Foo.
  class Bar {
    // Bar is based on the implementation from Foo.
    inherit ::this_program;

    // ...
  }

Note that this change is slightly incompatible with Pike 7.8.