Debian package: Add support for xulrunner 15 and 16

Debian package: Add support for xulrunner 15 and 16

Add support for xulrunner-15.0 and xulrunner-16.0 with according
versioned dependencies. Also add conflicts for earlier package
versions of xulrunner-15.0 and xulrunner-16.0 as them just being
unpacked will break the conkeror package (and not them). Also add
Lintian overrides for using Conflicts instead of Breaks.

xulrunner-14.0 is currently not supported as it has been already
released and getting the issue which has been fixed in xulrunner 15b3
and 16a2 in a released version is more work and therefore hasn't
happened yet. Not sure if it will happen at all.

Includes one minor optimization: debian/conkeror.bin (the wrapper
script which is installed as /usr/bin/conkeror) now uses `seq 13.0 -1
5.0` instead of `seq 5.0 13.0 | tac` in conkeror.bin. That saves one
fork and one redirection.