docs: Include files are always searched at standard location, too.

This removes the need to specify the location where yml2 was unpacked or
installed as YML_PATH. So this now works:

YML_PATH=/foo ./yml2proc --xml2yml

The standard system location <prefix>/share/yml2 is now always appended to
the include search path. If this path does not exist, the directory one
above the `backend.py` file is added, allowing the same behaviour for
uninstalled yml2.

In the Makefile enforce the .ysl2 files to be searched in the current
directory first.

list {
    address {
        name "Meier";
        street "Uhlmannstrasse 42";
        village "Laupheim";
    }
    address {
        name "Schmidt";
        street "Haupstrasse 23";
        village "Bad Waldsee";
    }
}