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.h

@@ -43,6 +43,8 @@ static inline int pidref_set_self(PidRef *pidref) {
         return pidref_set_pid(pidref, 0);
 }
 
+bool pidref_is_self(const PidRef *pidref);
+
 void pidref_done(PidRef *pidref);
 PidRef *pidref_free(PidRef *pidref);
 DEFINE_TRIVIAL_CLEANUP_FUNC(PidRef*, pidref_free);