socket_helper: set SO_KEEPALIVE on TCP sockets

socket_helper: set SO_KEEPALIVE on TCP sockets

Even LANs can break or be unreliable sometimes and socket
disconnect messages get lost, which means we fall back to
the global (kill -9) timeout in Unicorn.

While the default global timeout is much shorter (60s) than
typical TCP timeouts, some HTTP application dispatches take much
I/O or computational time (streaming many gigabytes), so the
global timeout becomes ineffective.

Under Linux, sysadmins are encouraged to lower the default
net.ipv4.tcp_keepalive_* knobs in sysctl.  There should be
similar knobs in other operating systems (the default keepalive
intervals are usually ridiculously high, too high for anything).

When the listen socket has SO_KEEPALIVE set, the flag should be
inherited by accept()-ed sockets.