(1) defs.${OPSYS}.mk --> platform/${OPSYS}.mk.
The "platform" subdirectory is where all of the ${OPSYS}-specific
infrastructure logic should reside.
(2) bsd.pkg.defaults.mk --> defaults/mk.conf
bsd.pkg.obsolete.mk --> defaults/obsolete.mk
Renaming bsd.pkg.defaults.mk to defaults/mk.conf is to mimic the way
that NetBSD has /etc/rc.conf as well as /etc/defaults/rc.conf, where
the latter is a full list of user-settable variables, and the two
files share the same name to reinforce the fact /etc/defaults/rc.conf
can be directly copied in place as /etc/rc.conf. This is the same
relationship shared by defaults/mk.conf and /etc/mk.conf.
