waitfor, uiwait and uiresume (was: On the performance of octave_usleep/gnulib::nanosleep on Win32)

[Michael Goffioul]

These changes implement the waitfor, uiwait and uiresume functions.
http://hg.savannah.gnu.org/hgweb/octave/rev/3b654a0753b1
http://hg.savannah.gnu.org/hgweb/octave/rev/ec435c4d8198 (correction
of the first one)

As discussed in the referenced thread, there's a performance penalty
in Win32 due to the nanosleep implementation in gnulib. I've reported
the issue on bug-gnulib
(http://lists.gnu.org/archive/html/bug-gnulib/2011-11/msg00302.html),
but there hasn't been any reaction so far. So I decided to submit the
change without the Win32 specific case (which I'll integrate into my
private patch).

Note also that these functions do not work with the gnuplot backend.

Michael.