mirb: Don't exit on Ctrl-C

mirb exits when I press Ctrl-C(SIGINT) during input.
I think it should go back prompt again, like irb.

 
I'll change like,
1. set signal handler before MIRB_READLINE().
2. MIRB_READLINE()
3. clear signal handler.

I'll also add,
```
#ifndef _WIN32
#define MIRB_SIGSETJMP(env) sigsetjmp(env, 1)
#define MIRB_SIGLONGJMP(env, val) siglongjmp(env, val)
#else
#define MIRB_SIGSETJMP(env) setjmp(env)
#define MIRB_SIGLONGJMP(env, val) longjmp(env, val)
#endif
```

I'll test longjmp() from signal handler works on Windows.