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/process-util.h

@@ -118,6 +118,10 @@ static inline bool ioprio_priority_is_valid(int i) {
         return i >= 0 && i < IOPRIO_BE_NR;
 }
 
+static inline bool pid_is_valid(pid_t p) {
+        return p > 0;
+}
+
 int ioprio_parse_priority(const char *s, int *ret);
 
 pid_t getpid_cached(void);