Extract the coverage sanitizer KPI to a new file.

This will allow multiple consumers of the coverage data to be compiled
into the kernel together. The only requirement is only one can be
registered

Extract the coverage sanitizer KPI to a new file.

This will allow multiple consumers of the coverage data to be compiled
into the kernel together. The only requirement is only one can be
registered at a given point in time, however it is expected they will
only register when the coverage data is needed.

A new kernel conflig option COVERAGE is added. This will allow kcov to
become a module that can be loaded as needed, or compiled into the
kernel.

While here clean up the #include style a little.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D18955

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