Install kernel debug data under /usr/lib/debug

This avoids needing a large boot partition / file system in order to
accommodate multiple kernels, and provides consistency with userland
debug. This also simplifies the process of moving kernel debug files
to a separate package and installing them on demand.

In addition, change kernel debug file extension to .debug, to match
userland debug files.

When using the supported kernel installation method the
/usr/lib/debug/boot/kernel directory will be renamed (to kernel.old)
as is done with /boot/kernel.

Developers wishing to maintain the historical behavior of installing
debug files in /boot/kernel/ can set KERN_DEBUGDIR="" in src.conf(5).

Reviewed by: bdrewery, brooks, imp, markj
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1006