Moving documentation

/usr/doc is accessed by users directly and we need to
guarantee a single canocical place for them to access
documentation, so we need another approach.

potato: use /usr/doc
woody: use /usr/share/doc and have symlinks in /usr/doc
woody+1: use /usr/share/doc exclusively
woody+2: drop /usr/doc completely

Make sure we deal with:
dpkg bug with replacing a directory with a symlink
systems where /usr/doc is a symlink to /usr/share/doc