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 "module" {
        | module «name(*)» {
        apply "*/interface";
        | };
    }

    template "interface" {
        | interface «name(*)» {
        apply "*/*", mode="func";
        | };
    }

    template "*", mode="func"
        | «name(.)» «name(*)»(`apply "*/*", mode="parms";`);

    template "*", mode="parms" {
        > «name(.)» «name(*)» «name(*/*)»
        if "position() != last()" > , 
    }
}