doc: update Doxyfile to doxygen 1.8.5

This is simply the result of "doxygen -u" on the previous configuration
and contains no functional changes. This eliminates warnings like:
Warning: Tag `SYMBOL_CACHE_SIZE' at line 310 of file Doxyfile has become obsolete.
when running "make doxygen". The changes are almost entirely in comments,
with obsolete settings removed and new settings added with their default values.