Generate README.Debian from patch README-Debian psuedo-headers
Rework debian/patch-to-news to generate the README.Debian entry from a
README-Debian DEP-3 style psuedo-header, when present. The header's
syntax is similar to the debian/control Description format. Among
other things, this change makes the common case of cherry-picking
upstream patches easier, and less invasive, since in many cases, a
simple README-Debian header can be appended while adding the other
headers, instead of (as previously) reindenting the upstream commit,
etc.