Copy head/@r262484 to projects/release-debugdist.

The purpose of this branch is to be able to set
WITH_DEBUG_FILES=1 for release builds so the *.debug.txz
distributions are accessible to the public, while keeping
the ISO sizes reasonably small.

There are going to be some likely disruptive changes for
this, so a project branch is likely necessary.

Sponsored by: The FreeBSD Foundation