[RFC] Page flags: Saving some, making handling easier etc.

A set of patches that attempts to improve page flag handling. First of all a
method is introduces to generate the page flag functions using macros. Then
the number of page flags used by sparsemem is reduced.

Then we add a way to export enum constant to the preprocessor which allows
us to get rid of __ZONE_COUNT and use the NR_PAGEFLAGS for the calculation
of actually available page flags for fields.

Lastly there is a land grab of page flags for various ongoing VM projects.

--