r17197: This patch moves the encryption of bulk data on SASL negotiated security

contexts from the application layer into the socket layer.

This improves a number of correctness aspects, as we now allow LDAP
packets to cross multiple SASL packets.  It should also make it much
easier to write async LDAP tests from windows clients, as they use SASL
by default.  It is also vital to allowing OpenLDAP clients to use GSSAPI
against Samba4, as it negotiates a rather small SASL buffer size.

This patch mirrors the earlier work done to move TLS into the socket
layer.

Unusual in this pstch is the extra read callback argument I take.  As
SASL is a layer on top of a socket, it is entirely possible for the
SASL layer to drain a socket dry, but for the caller not to have read
all the decrypted data.  This would leave the system without an event
to restart the read (as the socket is dry).

As such, I re-invoke the read handler from a timed callback, which
should trigger on the next running of the event loop.  I believe that
the TLS code does require a similar callback.

In trying to understand why this is required, imagine a SASL-encrypted
LDAP packet in the following formation:

+-----------------+---------------------+
| SASL  Packet #1 | SASL Packet #2      |
----------------------------------------+
| LDAP Packet #1       | LDAP Packet #2 |
----------------------------------------+

In the old code, this was illegal, but it is perfectly standard
SASL-encrypted LDAP.  Without the callback, we would read and process
the first LDAP packet, and the SASL code would have read the second SASL
packet (to decrypt enough data for the LDAP packet), and no data would
remain on the socket.

Without data on the socket, read events stop.  That is why I add timed
events, until the SASL buffer is drained.

Another approach would be to add a hack to the event system, to have it
pretend there remained data to read off the network (but that is ugly).

In improving the code, to handle more real-world cases, I've been able
to remove almost all the special-cases in the testnonblock code.  The
only special case is that we must use a deterministic partial packet
when calling send, rather than a random length.  (1 + n/2).  This is
needed because of the way the SASL and TLS code works, and the 'resend
on failure' requirements.

Andrew Bartlett
(This used to be commit 5d7c9c12cb2b39673172a357092b80cd814850b0)

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