Wrap the `boost::interprocess::file_lock` in a `std::unique_ptr` inside the map that keeps track of per-directory locks.

This fixes a build issue with the clang 4.0.0+boost-1.58.0p8 version combo on OpenBSD 6.2, and should have no effect otherwise.

Also add a unit test, make the function thread-safe, and fix Linux versus Windows behavior inconsistency.

Meant to fix #12413.