Use intptr_t and uintptr_t for integer/pointer conversions

Use intptr_t and uintptr_t for integer/pointer conversions

Use intptr_t and uintptr_t casts to appease the compiler when
values are converted between 32-bit integers and pointers.
This generates many warnings (currently ~170 on linux amd64)
when pointers are 64-bit.

These types are normally defined in stdint.h, and get defined
by autoconf if that's not the case.

In a few places, NULL is simply replaced with 0 where compared
with an integer.

Reviewed-on: http://gerrit.openafs.org/474
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>