Implement the signbit() function

[John W. Eaton]

On  3-Oct-2012, Michael D. Godfrey wrote:

| I looked some more at mappers.cc, but could not
| understand it well enough to be confident about
| implementing a signbit() function whose behavior would
| be exactly like sign() but it would return the signbit(s)
| of its argument, and it can directly use the signbit in C++.
| Implementing this function should be
| fairly easy for someone with a good understanding
| of mappers.cc.
| 
| The most obvious place where signbit() would be useful,
| for me, is when using atan2.  The sign is needed to decide
| on the correct quadrant where atan2 is +- 0..  Doing this now
| is definitely cumbersome.  It is easy to think of other applications.
| 
| So, if you agree that this is useful, can we find someone to
| do it?

I checked in the following change:

  http://hg.savannah.gnu.org/hgweb/octave/rev/ecf5be238b4a

jwe