Clean shutdown with SIGINT

[foxbunny]

FSAL currently isn't able to shut down cleanly on keyboard interrupt. When interrupt is received, the following traceback is printed before server quits:

Traceback (most recent call last):
  File "/home/hajime/.envs/libtest/bin/fsal", line 9, in <module>
    load_entry_point('fsal==0.0.1', 'console_scripts', 'fsal')()
  File "/home/hajime/.envs/libtest/lib/python2.7/site-packages/fsal/server.py", line 126, in main
    server.run()
  File "/home/hajime/.envs/libtest/lib/python2.7/site-packages/fsal/server.py", line 83, in run
    server.serve_forever()
  File "/usr/lib64/python2.7/contextlib.py", line 35, in __exit__
    self.gen.throw(type, value, traceback)
  File "/home/hajime/.envs/libtest/lib/python2.7/site-packages/fsal/server.py", line 110, in open_socket
    sock.shutdown(socket.SHUT_RDWR)
  File "/home/hajime/.envs/libtest/lib/python2.7/site-packages/gevent/socket.py", line 519, in shutdown
    self._sock.shutdown(how)
socket.error: [Errno 9] Bad file descriptor

(Tested with code in a_hand branch).