diff: exit with status 1, not 2, when binary files differ

Problem reported by Vincent Lefevre in <http://bugs.gnu.org/16608>.
* NEWS:
* doc/diffutils.texi (Binary, Invoking diff): Document this.
* src/analyze.c (briefly_report): Return void, not int.
All uses changed.  Do not futz with exit status.  Simplify.
* tests/binary: Adjust to match new behavior.