Move cpus_in_affinity_mask() to cpu-set-util.[ch]

It just seems to fit better there and it's always better to have things
in shared/ rather than basic/.

src/basic/process-util.h

@@ -194,5 +194,3 @@ assert_cc(TASKS_MAX <= (unsigned long) PID_T_MAX)
                 (pid) = 0;                      \
                 _pid_;                          \
         })
-
-int cpus_in_affinity_mask(void);