pidref: add new pidref_is_self() helper

This simply checks if the specified PidRef refers to the process we are
running in.

(In case you wonder why this is not a static inline: to avoid cyclic
header inclusion problems between pidref.h + process-util.h)

src/basic/pidref.c

@@ -263,6 +263,13 @@ int pidref_verify(const PidRef *pidref) {
         return 1; /* We have a pidfd and it still points to the PID we have, hence all is *really* OK → return 1 */
 }
 
+bool pidref_is_self(const PidRef *pidref) {
+        if (!pidref)
+                return false;
+
+        return pidref->pid == getpid_cached();
+}
+
 static void pidref_hash_func(const PidRef *pidref, struct siphash *state) {
         siphash24_compress(&pidref->pid, sizeof(pidref->pid), state);
 }