Fix debian watch file

debian/watch

 version=3
-opts=pgpsigurlmangle=s/$/.asc/,downloadurlmangle=s/(.*)/$1\/awit-dbackup-$1\.tar\.gz/ \
-http://download.allworldit.org/awit-dbackup/v(.+)
+opts=pgpsigurlmangle=s/$/.asc/,\
+downloadurlmangle=s/\/(v?\d+\.\d+\.\d+.*)$/\/$1\/awit-dbackup-$1.tar.xz/,\
+filenamemangle=s/v?(\d+\.\d+\.\d+.*)$/awit-dbackup-$1.tar.xz/ \
+http://download.dbackup.org/v?(\d+\.\d+\.\d+.*)