border render support through plugin. Working with current yafray CVS version

Multi-cpu support in yafray is back. This is a small fix to pass the
number of processors to yafray through plugin too. Will work with current
CVS yafray code. Now at p.b.org. Needs testing with a real multicpu machine.

Fixed execution bit test for yafray plugin loading under unix.
Now, it just tests for read permissions.

cache_size yafray parameter calculation changed to sync with current
yafray cvs code. Now given in screen coords (-1,+1)x(-1,+1)

Finally, fixed the evil dll problem. It works now
on my win32 box with my tests without crashing.
But be aware! There may be more of them watching
us, waiting for a moment of distraction, nobody
is safe.

minor changes to sync with yafray cvs interface, getting closer
to the clean msvc build.

These are changes sent by Luis_F who got this working on win32. Only applies
to that platform and fixes a problem findind yafray dll's

NaN makefile fix for yafray dir and typo corrected on "xml" button tooltip

These are changes necessary for building with msvc in a near future.
Just making sure memory is always allocated and freed in the same
binary object, which seems to be mandatory on msvc if you don't have
the comercial version. If you are using this code you need to
syncronize with yafray cvs also, or it won't work. Binaries for os x
are updated in the link I gave in my last commit.

New export through plugin scheme for yafray. It is working on linux and mac os x
Needs latest yafray, you can get it from cvs, but I have also binaries
for os x here:

http://www.coala.uniovi.es/~jandro/noname/downloads/yafray-0.0.6-3.pkg.zip

To use it, go to yafray panels (global settings) and uncheck the "xml" button.
That would tell the export code to avoid xml export and use the yafray plugin
instead. You'll see the render being draw while running and you can even stop it
with ESC key.

Since I'm sure problems will appear, expect updates soon.
Remember: does not work on win32