Fix re-used connections after fork (again)

[stefanmh]

When fork()ing a process having a Redis object, the child inherits the file descriptors.
To prevent a race, the child correctly refuses to use the inherited connections (see b8dd224). It thus calls close() on those fd-s (good), but also shutdown() (bad), thus breaking the socket for the parent.

See e.g. http://www.unixguide.net/network/socketfaq/2.6.shtml:

Generally the difference between close() and shutdown() is: close()
closes the socket id for the process but the connection is still
opened if another process shares this socket id. The connection stays
opened both for read and write, and sometimes this is very important.
shutdown() breaks the connection for all processes sharing the socket
id. Those who try to read will detect EOF, and those who try to write
will reseive SIGPIPE, possibly delayed while the kernel socket buffer
will be filled.

[rolette]

Same issues as #732. I submitted a fix for it (#784) that we've been running for months, but it hasn't been merged in.

[andymccurdy]

This is now resolved. Sorry for the amount of time that it took.

[andymccurdy]

3.2.0 has been released with these fixes.