nbd/client: Split handshake into two functions

An upcoming patch will add the ability for qemu-nbd to list
the services provided by an NBD server.  Share the common
code of the TLS handshake by splitting the initial exchange
into a separate function, leaving only the export handling
in the original function.  Functionally, there should be no
change in behavior in this patch, although some of the code
motion may be difficult to follow due to indentation changes
(view with 'git diff -w' for a smaller changeset).

I considered an enum for the return code coordinating state
between the two functions, but in the end just settled with
ample comments.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190117193658.16413-15-eblake@redhat.com>