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

110 DATA "Meier", "Uhlmannstrasse 42", "Laupheim"

     2

120 DATA "Schmidt", "Haupstrasse 23", "Bad Waldsee"

     3

1000 FOR I=1 TO 2 STEP 1

     4

1010 READ NAME$, STREET$, VILLAGE$

     5

1020 PRINT "INSERT INTO address (name, street, village) VALUES ('" + NAME$ + "', '" + STREET$ + "', '" + VILLAGE$ + "');"

     6

1030 NEXT I