Fixed data install dir and data dir lookup

Previously the openxcom data was installed in /usr/share/openxcom/ and
not the correct /usr/share/openxcom/data/ dir when doing "make install".
This is now fixed.

You can now also have data spread out in the valid list of data
dirs. For example the openxcom data files in /usr/share/openxcom/data/
and the original xcom datafiles in ~/.local/share/openxcom/data