[client] Switch to non-blocking sockets
    
    Switch the gssproxy client library to non-blocking sockets, allowing
    for timeout and retry operations.  The client will automatically retry
    both send() and recv() operations three times on ETIMEDOUT.  If the
    combined send() and recv() hit the three time limit, ETIMEDOUT will be
    exposed to the caller in the minor status.
    
    Signed-off-by: Alexander Scheel <ascheel@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>
    [rharwood@redhat.com: commit message cleanups, rebased]
    Reviewed-by: Robbie Harwood <rharwood@redhat.com>