Perform an actual access check in Dir.tmpdir for writability

At the moment, this code is looking at the stat output to determine if a
temp directory can be written to. However, just because the stat bits
say that a directory is writable, does not make it so; and, likewise,
the operating system may in fact grant access to paths that the stat
bits and process UID say should be inaccessible.

These systems include:

* Posix ACL's
* Linux's capabilities like CAP_DAC_OVERRIDE
* Linux Security Modules like SELinux or AppArmor
* Syscall filters like Linux's seccomp
* Granular capability systems like FreeBSD's Capsicum
* OpenBSD's pledge and unveil
* Windows too has a rich ACL system for controlling filesystem access

The best thing to do is simply to try and access the path with
`File.writable?` and let the operating system tell us if the path can be
accessed.

lib/tmpdir.rb

@@ -36,7 +36,9 @@ class Dir
       case
       when !stat.directory?
         warn "#{name} is not a directory: #{dir}"
-      when !stat.writable?
+      when !File.writable?(dir)
+        # We call File.writable?, not stat.writable?, because you can't tell if a dir is actually
+        # writable just from stat; OS mechanisms other than user/group/world bits can affect this.
         warn "#{name} is not writable: #{dir}"
       when stat.world_writable? && !stat.sticky?
         warn "#{name} is world-writable: #{dir}"