build(make): add deb and arch targets and a staged install

install now honors DESTDIR and also installs the license and README under DOCDIR, so packagers can stage a filesystem tree. Add deb (runs dpkg-buildpackage, collects the artifacts in dist/) and arch (runs makepkg) targets, plus uninstall. Ignore the Debian and Arch build output.

.gitignore

@@ -19,3 +19,15 @@ coverage.*
 *.swp
 *~
 .DS_Store
+
+# Debian packaging build output
+/debian/files
+/debian/lazyntfy/
+/debian/*.substvars
+/debian/.debhelper/
+
+# Arch Linux packaging build output
+/packaging/archlinux/src/
+/packaging/archlinux/pkg/
+/packaging/archlinux/*.pkg.tar.*
+/packaging/archlinux/*.tar.gz