Debian package: Make use of upstream's rewritten Makefile

Debian package: Make use of upstream's rewritten Makefile

Previously Conkeror's Makefile had no install target. So
debian/conkeror.install, debian/conkeror.manpages contained a list of
files and directories which needed to be installed into the Debian
package.

With the Makefile's install target, ebian/conkeror.manpages is
obsolete and debian/conkeror.install could be greatly reduced. The
latter now only states that the contents of /usr/share and some
debian-specific files are needed.

Since the Makefile's install target also takes care of directory
creation under the installation target directory, debian/conkeror.dirs
and debian/conkeror-spawn-process-helper.dirs are now obsolete and
removed, too.

The only thing which had to be added was a small blacklist of files to
remove again from the Debian package after "make install" has been run:

* COPYING (not needed as Debian ships a copyright file in a machine
  readable format)
* Info.plist (only useful on MacOS X)

This blacklist is now hardcoded in the debian/rules file in the
override_dh_auto_install target.