xserver warning fixes

[Jamey Sharp]

Here is a collection of simple fixes for the warnings I see when
building xserver on my system. I'm building on 64-bit Linux with GCC, so
several of these patches are for warnings that are 64-bit specific, and
some of the fixes suppress warnings in GCC-specific ways (but should
still be correct on other compilers).

None of these patches depend on each other, so they can be applied in
any order or dropped, as appropriate.

With these patches and the libxtrans patch I posted earlier, I can build
xserver commit 4c6bfa2c09ae2b0cffdf9211a6dfbcaefe0366b5 with -Werror,
except for two warnings:

hw/xfree86/os-support/linux/lnx_agp.c:267: warning: cast to pointer from
integer of different size
hw/xfree86/dri/dri.c:431: warning: cast from pointer to integer of
different size

Both of these cases look like potentially serious API bugs to me, and I
don't know what to do about them.

Jamey