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