linuxkpi.4: extend for compilation and tracking consumers

[bz]

Summary

Extend linuxkpi.4 adding kenrel compile time options,
information on how to compile a driver into the kernel or how to
compile as a module. Add a sysctl section. Rework other bits from
the initial "dummy" page.

MFC after: 3 days

[bz]

Please note there are a lot of commented out parts in there for drivers, etc. which do use LinuxKPI, which are in the tree, but not yet hooked up to the build.