After e284f3ff6831, ngx_crypt() can no longer return NGX_AGAIN.

Found by gcc7 (-Wimplicit-fallthrough).

Previously, the source IP address of a response UDP datagram could differ from
the original datagram destination address.  This could happen if the server UDP
socket is bound to a wildcard address and the network interface chosen to output
the response packet has a different default address than the destination address
of the original packet.  For example, if two addresses from the same network are
configured on an interface.

Now source address is set explicitly if a response is sent for a server UDP
socket bound to a wildcard address.

This change allows setting the destination IPv6 address of a UDP datagram
received on a wildcard socket.

The ngx_linux_sendfile() function is now used for both normal sendfile()
and sendfile in threads.  The ngx_linux_sendfile_thread() function was
modified to use the same interface as ngx_linux_sendfile(), and is simply
called from ngx_linux_sendfile() when threads are enabled.

Special return code NGX_DONE is used to indicate that a thread task was
posted and no further actions are needed.

If number of bytes sent is less that what we were sending, we now always
retry sending.  This is needed for sendfile() in threads as the number
of bytes we are sending might have been changed since the thread task
was posted.  And this is also needed for Linux 4.3+, as sendfile() might
be interrupted at any time and provides no indication if it was interrupted
or not (ticket #1174).

This has somehow escaped from fbdaad9b0e7b.