Addresses a Debian lintian complaint. Adds a debian/watch file for Debian's
automated version-tracking tools. This requires new upstream versions to be
tagged, since the version number is pulled from the filenames of tarballs
available on github.

Addresses a Debian lintian complaint. Changed the format line in the
debian/copyright file from its proposed form (DEP 5) to the location of
the accepted standard.

Otherwise the variable is unused, causing a compiler warning

“If two pointers p and q of the same type point to different that are
not members of the same object or elements of the same array or to
different functions, or if only one of them is null, the results of
p<q, p>q, p<=q, and p>=q are unspecified.”

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Closes #28 github issue.