build system: New "make dist" uses git; add release checklist

We no longer try to separately track which files should be shipped in
the tarball.  Instead, the tarball contains exactly the same files as
the repo.

Add a release checklist at the bottom of Makefile.in, based on the one
from the stable branch (in 813ff677f7020b12fb1356e977a73dc910e64ce5).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>