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.

     1

decl package +name;

     2

decl class +name;

     3

decl attr +type +name;

     4

     5

package Bla {

     6

    class Customer {

     7

        attr long id;

     8

        attr string name;

     9

    }

    10

    class Order {

    11

        attr long id;

    12

        attr string desc;

    13

    }

    14

}

    15