Now code generation can handle zero-offset fixed tags with generic API.

Now code generation can handle zero-offset fixed tags with generic API.

We'll need this for POSIX captures: in the prsence of orbit tag,
opening tag is needed only for disambiguation; after determinization
we can pretend that it is fixed on orbit tag with zero offset.
This will save some time and space on tag optimizations and maybe
reduce the number of tag variables.