web: Update Doxyfile to doxygen-1.8.6.

web: Update Doxyfile to doxygen-1.8.6.

The system on which the public web pages are generated is now running
on Ubuntu-14.04 which ships doxygen-1.8.6. This version prints a
couple of warnings about obsolete tags in Doxyfile:

	Warning: Tag `SYMBOL_CACHE_SIZE' at line 289 of file - has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
	Warning: Tag `SHOW_DIRECTORIES' at line 483 of file - has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
	Warning: Tag `HTML_ALIGN_MEMBERS' at line 820 of file - has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
	Warning: Tag `USE_INLINE_TREES' at line 989 of file - has become obsolete.
	To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

This patch is the result of running doxygen -u. With the patch applied,
the above warnings go away.

There is no need to be backwards compatible here because the web pages
are not created during a normal build.