Fixes to the nonce code

Fixes to the nonce code

On Unix, we create a 700 subdir in /tmp only readable by the user, on
Windows we use the user-specific tmpdir and create the noncefile
directly.

Add NonceFile to abstract this behaviour and to delete the noncefile
and possibly tmpdir on shutdown.

Cherry-picked from commit 885f16b90c4f769ae29f432d0ed2a63bb2e4dab8 in
the dbus4win repository. Fixed to apply and correct whitespace issues
by tml@iki.fi.