DBusNonceFile: don't always fail when use_subdir=TRUE, i.e. on Unix

DBusNonceFile: don't always fail when use_subdir=TRUE, i.e. on Unix

In shell-like pseudocode, the desired result is:

if $use_subdir
  dir = $tmpdir/dbus_nonce-$random
  path = $dir/nonce
  mkdir $dir
  write file at $path
else
  dir = $user_owned_place
  path = $dir/dbus_nonce-$random
  write file at $path

However, /nonce was accidentally appended to $dir instead of $path,
resulting in an attempt to mkdir /tmp/dbus_nonce-XXXX/nonce when
dbus_nonce-XXXX hadn't been created yet.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=34569
Tested-by: David Zeuthen <davidz@redhat.com>
Reviewed-by: David Zeuthen <davidz@redhat.com>