debian: create dbgsym packages if debhelper supports them

debian: create dbgsym packages if debhelper supports them

Starting with Debian Stretch, we now have support for dbgsym packages
which are uploaded to a separate package archive.  Since some people
might want to try to backport e2fsprogs 1.43 for Jessie set things up
so that the package can be built on older Debiain systems (where the
legacy dbg packages will be created) as well as newer (as of this
writing, Debian Sid or Stretch).

Signed-off-by: Theodore Ts'o <tytso@mit.edu>