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.

src/basic/macro.h

@@ -298,6 +298,9 @@ static inline unsigned long ALIGN_POWER2(unsigned long u) {
 #define PTR_TO_GID(p) ((gid_t) (((uintptr_t) (p))-1))
 #define GID_TO_PTR(u) ((void*) (((uintptr_t) (u))+1))
 
+#define PTR_TO_PID(p) ((pid_t) ((uintptr_t) p))
+#define PID_TO_PTR(p) ((void*) ((uintptr_t) p))
+
 #define memzero(x,l) (memset((x), 0, (l)))
 #define zero(x) (memzero(&(x), sizeof(x)))