limits.h: provide all limit macros for standard [u]intNN_t types

limits.h: provide all limit macros for standard [u]intNN_t types

Currently, we only have UINT32_MAX and UINT64_MAX in limits.h, and
then stdint.h and kernel.h somewhat randomly define UINT8_MAX and
INT32_MAX, respectively.

Provide a full set of definitions in terms of the min/max macros for
the types that [u]intNN_t are defined in terms of, namely the {s,u}NN
ones.

Try to avoid breaking whatever depended on getting UINT8_MAX from our
compat stdint.h by replacing it with an include of limits.h.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>