Build and install userland .debug files by default

Debug data files are now built by default with 'make buildworld' and
installed with 'make installworld'. This facilitates debugging but
requires more disk space both during the build and for the installed
world. Debug files may be disabled by setting WITHOUT_DEBUG_FILES=yes
in src.conf(5).

Reviewed by: bdrewery, eadler, vangyzen
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4018