vcl: improve shutdown()

vcl: improve shutdown()

This commit does following:

- Change the behavior of shutdown() with SHUT_RDWR flag.
- Check SHUT_RD flag when read()
- Change the errno when write() after SHUT_WR
- Remove unused code

All the above modification passed the packetdrill test.

Type: improvement

Signed-off-by: liuyacan <liuyacan@corp.netease.com>
Change-Id: I0c81f52e563562e58580d70976526b898e65e915