[C++0x] nullptr

[Benjamin Kosnik]

> additionally to Jonathan's comments, I don't understand this member
> typedef, I suspect should simply go away:

...yeah, I don't understand why nullptr_t as defined in cstddef isn't
just being used instead of using the (oft-repeated) __nullptr_t type.

This applies to shared_ptr/unique_ptr/functional.

This will be more interesting to look at this when the patches + trunk
==  bootstrap. I suspect the subtle things WRT compat and overloading
will be most easily revealed in the testsuite.

-benjamin