macro: introduce new PID_TO_PTR macros and make use of them

macro: introduce new PID_TO_PTR macros and make use of them

This adds a new PID_TO_PTR() macro, plus PTR_TO_PID() and makes use of
it wherever we maintain processes in a hash table. Previously we
sometimes used LONG_TO_PTR() and other times ULONG_TO_PTR() for that,
hence let's make this more explicit and clean up things.