Update atf to 0.18:

Experimental version released on November 16th, 2013.

* Issue 45: Added require.memory support in atf-run for FreeBSD.

* Fixed an issue with the handling of cin with libc++.

* Issue 64: Fixed various mandoc formatting warnings.

* NetBSD PR bin/48284: Made atf-check flush its progress message to
  stdout so that an interrupted test case always shows the last message
  being executed.

* NetBSD PR bin/48285: Fixed atf_check examples in atf-sh-api(3).

(jmmv)