Add fix for "path::lexically_relative is confused by trailing slashes".

	* doc/xml/manual/intro.xml: Document LWG 3096 change.
	* src/filesystem/std-path.cc (path::lexically_relative(const path&)):
	Treat a final empty element equivalently to a final dot element.
	* testsuite/27_io/filesystem/path/generation/relative.cc: Add checks
	for the examples in the DR.

From-SVN: r266566