Add debian packaging files

This doesn't quite work, since the debuild stuff refuses to see the
rustup-installed cargo/rustc, which is required for our build.

debian/copyright

@@ -0,0 +1,26 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: bscreensaver
+Upstream-Contact: Brian Tarricone <brian@tarricone.org>
+Source: https://github.com/kelnos/bscreensaver
+
+Files: *
+Copyright: 2022 Brian Tarricone <brian@tarricone.org>
+License: GPL-3
+
+License: GPL-3
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+ .
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+ .
+ You should have received a copy of the GNU General Public License
+ along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ .
+ On Debian systems, the full text of the GNU General Public
+ License version 3 can be found in the file
+ '/usr/share/common-licenses/GPL-3'.