core: in execute, Never fail setting Nice priority

Instead, push to the closest possible Nice priority setting.

Replaces: #11397

src/basic/process-util.h

@@ -200,3 +200,5 @@ assert_cc(TASKS_MAX <= (unsigned long) PID_T_MAX);
         })
 
 int pidfd_get_pid(int fd, pid_t *ret);
+
+int setpriority_closest(int priority);