Undeprecate unique_path(). Instead, add a note mentioning the workaround for lack of thread safety and possible change to cwd. unique_path() is just too convenient to deprecate!

[SVN r67571]

libs/filesystem

@@ -1 +1 @@
-Subproject commit 76ae0363fb4c183233c0d81be35236d95089b12e
+Subproject commit cc697684a183d6eedd6d283ecea1d86892fc9caa