[Mono-winforms-list] Remove horrible MONO_MWF_USE_CARBON_BACKEND env vars

[Geoff Norton]

Attached is a few patches (1 against mcs for Sys.D and MWF; 1 against  
gdi+) that does a few things:

#1 Properly determine dpi on the mac
#2 Remove all usage of MONO_MWF_USE_CARBON_BACKEND and refactor it to  
using uname os detection
#3 Remove all usage of MONO_MWF_USE_COCOA_BACKEND as its deprecated,  
unsupported and unworking.

This changes the default behaviour to use the carbon backend on the  
mac in all cases.  Thoughts?  Comments?

We could possible provide a env override to force back to X11 on the  
mac, but since we dont work on 10.5 anyways (x11) I'm not sure how  
much value there is in this.

I'll be commiting tomorrow unless people have objections.
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ps> tested on Leopard x86, Leopard ppc, openSUSE 10.3 x86_64; no  
regressions

-g