Separate child information into a new structure

[hangpark]

syscall_exec() waits a new process to be loaded by calling thread_yield() repetitively. While this waiting, a new process would be executed to load its data on the memory. But, it is also possible to happen that the new process terminates (after loaded and executed) before the parent wakes up. If so, child process was freed, so we can not deal with child process' information during the parent's last of syscall_exec() executions:

/* Return PID. */
if (p->status & PROCESS_FAIL)
    return PID_ERROR;
return p->pid;

Fortunately, Bochs does not involve this situation, but Qemu does (in such as wait-simple test, etc.)

We can avoid the situation in general making child process' information in another struct so the parent can manage it even if the child process is terminated and freed.

[hangpark]

Done by PR #72.