Currently, the current line is not cleared by Cltr-C

After this patch
```
>>> asdfasdf^C
Traceback (most recent call last):
  File "~/spark/python/pyspark/context.py", line 225, in signal_handler
    raise KeyboardInterrupt()
KeyboardInterrupt
```

It's still worse than 1.5 (and before).

Author: Davies Liu <davies@databricks.com>

Closes #10134 from davies/fix_cltrc.