cpplib: simplify callbacks, reduce exposed interface

[Neil Booth]

Zack Weinberg wrote:-

> I'd support this, and also removing the ability to specify trailing
> flags on #line.  As you say, it can probably cause segfaults and the
> like.  It used to be useful to write #line "foobar" 3, but now we have
> #pragma GCC system_header for that.  

Right, but we reject that in the main file.

What should I do to update the test suites?  Remove all the tests for
no warnings in system headers?  Or update cpp to accept the #pragma
even in the main file?

Neil.