pidref: now that we have the cached pidfdid of our own process, use it

Note that this drops a lot of "const" qualifiers on PidRef arguments.
That's because pidref_is_self() suddenly might end changing the PidRef
because it acquires the pidfd ID.

We had this previously already with pidfd_equal(), but this amplifies
the problem.

I guess we C's "const" doesn't really work for stuff that contains
caches, that is just conceptually constant, but not actually.

src/basic/process-util.c

@@ -1120,7 +1120,7 @@ int getenv_for_pid(pid_t pid, const char *field, char **ret) {
         return 0;
 }
 
-int pidref_is_my_child(const PidRef *pid) {
+int pidref_is_my_child(PidRef *pid) {
         int r;
 
         if (!pidref_is_set(pid))
@@ -1150,7 +1150,7 @@ int pid_is_my_child(pid_t pid) {
         return pidref_is_my_child(&PIDREF_MAKE_FROM_PID(pid));
 }
 
-int pidref_is_unwaited(const PidRef *pid) {
+int pidref_is_unwaited(PidRef *pid) {
         int r;
 
         /* Checks whether a PID is still valid at all, including a zombie */