This rationalises atari platform user of headers and squashes a lot of
warnings. Should not be viewed as real maintenance, more that it was
getting in the way of fixing real core header issues.
The netsurf.h header should *only* contain the registration, core
initialisation and finalisation methods. Version information is best
placed in its own header.
Also remove any unneeded inclusion of this header limiting it to
solely the places the relevant API is required.
