fix some vDSO bugs.

- vDSO sometimes becomes invalid.
- vDSO is not succeeded for child process.
- vDSO becomes invalid when execve.
refs #744

executer/user/mcexec.c

@@ -2198,6 +2198,7 @@ fork_err:
 						strcpy(desc->shell_path, shell_path);
 					}
 
+					desc->enable_vdso = enable_vdso;
 					__dprintf("execve(): load_elf_desc() for %s OK, num sections: %d\n",
 						path, desc->num_sections);