TCP Fast Open support

[oakaigh]

@dlundquist Users can enable TCP Fast Open for both listening (server/frontend) and client (backend) sockets by adding one line to the listen section

listen * {
    # ...
    # frontend - fastopen server
    # backend - fastopen client
    # yes - fastopen client and server
    fastopen yes
    # ...
}

[oakaigh]

@dlundquist Please ignore changes in

redhat/* configure.ac

[dlundquist]

Note to self: see #294 and #308 for context.