parse-util: introduce pid_is_valid()

Checking for validity of a PID is relatively easy, but let's add a
helper cal for this too, in order to make things more readable and more
similar to uid_is_valid(), gid_is_valid() and friends.

src/basic/parse-util.c

@@ -59,7 +59,7 @@ int parse_pid(const char *s, pid_t* ret_pid) {
         if ((unsigned long) pid != ul)
                 return -ERANGE;
 
-        if (pid <= 0)
+        if (!pid_is_valid(pid))
                 return -ERANGE;
 
         *ret_pid = pid;