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 yslt.yml2

tstylesheet {
    template "/list/*"
        | INSERT INTO «name()» (`apply "*", 0, mode="names"`) VALUES (`apply "*", mode="values"`);
    
    template "*", mode="names" > «name()»`if "position()!=last()" ", "`
    template "*", mode="values" > '«.»'`if "position()!=last()" ", "`
}