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

In order to match the other get_process_xyz() calls.

src/test/test-process-util.c

@@ -55,7 +55,7 @@ static void test_get_process_comm(void) {
         assert_se(get_process_cmdline(1, 8, false, &d) >= 0);
         log_info("pid1 cmdline truncated: '%s'", d);
 
-        assert_se(get_parent_of_pid(1, &e) >= 0);
+        assert_se(get_process_ppid(1, &e) >= 0);
         log_info("pid1 ppid: "PID_FMT, e);
         assert_se(e == 0);