Added DoD Distribution Statement A.

Added DoD Distribution Statement A, which pertains to DARPA's support for
certain portions of the Emulab software.  The statement says that these
portions of Emulab are "approved for public release; distribution is
unlimited."

Also performed other various cleanups: put two blank lines before headings;
indent displayed URLs and email addresses consistently; fixed a typo in the
emulab-users list URL; and fixed punctuation around quotes.