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.

include uml.yml2

UML {
    include uml2sql.yml2

    Root applyPackages;

    Package {
        param "name", "''";
        if "$name=''"  apply "packagedElement", 0 { with "name", "@name"; }
        if "$name!=''" apply "packagedElement", 0 { with "name", "concat($name, '_', @name)"; }
    }

    Class {
        param "name";

        | CREATE TABLE «$name»_«@name» (
        applyAttributes;
        | );
    }

    Attribute | «@name» †type`if "position()!=last()" > ,`
}