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

list {

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    address {

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        name "Meier";

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        street "Uhlmannstrasse 42";

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        village "Laupheim";

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    }

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    address {

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        name "Schmidt";

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        street "Haupstrasse 23";

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        village "Bad Waldsee";

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    }

    12

}