Fix {Ignore, RetryOn}Signals on socket.error

Some confusion arose handling EINTR on those functions: in python 2.6
socket.error is an IOError, and thus:
  - It's an EnvironmentError
  - It has an .errno member

In 2.4 and 2.5 it's not, and so its errno variable must be extracted
from the args tuple. This patch fixes both functions, and the unittests.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>