process-util: rename get_parent_of_pid() → get_process_ppid()

In order to match the other get_process_xyz() calls.

src/basic/process-util.c

@@ -402,7 +402,7 @@ int get_process_environ(pid_t pid, char **env) {
         return 0;
 }
 
-int get_parent_of_pid(pid_t pid, pid_t *_ppid) {
+int get_process_ppid(pid_t pid, pid_t *_ppid) {
         int r;
         _cleanup_free_ char *line = NULL;
         long unsigned ppid;