Merge ^/projects/release-debugdist into ^/head:
r262491, r262493, r262516, r267345, r267397:

r262491:
  Add DEBUG_DISTRIBUTIONS, and set it to include base and
  EXTRA_DISTRIBUTIONS, excluding 'doc', since the documentation
  distribution does not have corresponding debug information.

  Use DEBUG_DISTRIBUTIONS in the 'distributeworld installworld'
  and 'packageworld' targets, to reduce the number of occurances
  of excluding distributions that do not have .debug files.

r262493:
  In release/Makefile, explicitly set WITHOUT_DEBUG_FILES=1
  for dvdrom and cdrom targets.  (Later reverted.)

  Exclude the *.debug.txz distributions from dvdrom and
  cdrom images, but include them for ftp distribution.

r262516:
  Rename ${dist}.debug.txz to ${dist}-dbg.txz to prevent the
  following output:
	eval: ${base....}: Bad substitution
	eval: ${doc....}: Bad substitution
	eval: ${games....}: Bad substitution
	eval: ${lib32....}: Bad substitution

  This also follows other naming conventions seen in the
  wild.

r267345:
  Explicitly set MK_DEBUG_FILES=no, which overrides the
  WITH_DEBUG_FILES=1 and WITHOUT_DEBUG_FILES=1 collisions
  previously experienced.

  This change allows us to create the {base,kernel}_debug.txz
  distributions without accidentally installing the *.debug
  files on the medium itself.

r267397:
  Remove evaluations of MK_DEBUG_FILES where not needed.
  If DEBUG_DISTRIBUTIONS is empty, which is true if
  MK_DEBUG_FILES evaluates to 'no' above, the loop does
  nothing.

MFC after:	1 month
Tested on:	head@r267801
Reviewed by:	brooks [1], emaste, imp [1]
		[1] earlier version
Sponsored by:	The FreeBSD Foundation