Squashed commit of the following:

Squashed commit of the following:

Closes: gh-90

commit 2e7b8e0d649267d2824de0f1d0c02fa23125219b
Author: Austin S. Hemmelgarn <ahferroin7@gmail.com>
Date:   Mon Nov 26 10:32:02 2018 -0500

    Simplify listening socket creation logic.

    As suggested by @yoe.

commit 73518c1a931e3a309d5233d562cc53c683bcdc88
Author: Austin S. Hemmelgarn <ahferroin7@gmail.com>
Date:   Thu Nov 15 09:25:22 2018 -0500

    nbd-server: Add an option to listen on both UDS and TCP/SDP

    This adds a configuration parameter `duallisten` for nbd-server.  When
    true and a unix socket is specified, this will cause nbd-server to also
    listen on any TCP/SDP sockets it would have listened on if the unix
    socket had not been specified.

commit 3dd8449db6ef6198813230026879c0df0c087e7e
Author: Austin S. Hemmelgarn <ahferroin7@gmail.com>
Date:   Fri Nov 9 10:08:55 2018 -0500

    Don't listen on TCP if we are using a UNIX socket.

    Currently, `nbd-server` will open a listening TCP socket even if it's
    been asked to use a UNIX domain socket.  This is inconsistent with
    typical behavior of applications which can use UNIX sockets, and may
    present a security risk on some systems.

    This commit modifies this behavior so that use of a UNIX socket and
    other types of socket is mutually exclusive.  You can only listen on a
    regular (TCP or SDP) socket or a UNIX socket, but not both, with the
    prescense of the `unixsock` option taking precendence over any other
    listener configuration.

    This has a small potential to break some existing installations, though
    they are arguably depending on broken behavior to begin with.

    The manual page is also updated to reflect this change.

    Signed-off-by: Austin S. Hemmelgarn <ahferroin7@gmail.com>