pgsql: Improve the SASL authentication protocol.

Improve the SASL authentication protocol.

This contains some protocol changes to SASL authentiation (which is new
in v10):

* For future-proofing, in the AuthenticationSASL message that begins SASL
authentication, provide a list of SASL mechanisms that the server
supports, for the client to choose from. Currently, it's always just
SCRAM-SHA-256.

* Add a separate authentication message type for the final server->client
SASL message, which the client doesn't need to respond to. This makes
it unambiguous whether the client is supposed to send a response or not.
The SASL mechanism should know that anyway, but better to be explicit.

Also, in the server, support clients that don't send an Initial Client
response in the first SASLInitialResponse message. The server is supposed
to first send an empty request in that case, to which the client will
respond with the data that usually comes in the Initial Client Response.
libpq uses the Initial Client Response field and doesn't need this, and I
would assume any other sensible implementation to use Initial Client
Response, too, but let's follow the SASL spec.

Improve the documentation on SASL authentication in protocol. Add a
section describing the SASL message flow, and some details on our
SCRAM-SHA-256 implementation.

Document the different kinds of PasswordMessages that the frontend sends
in different phases of SASL authentication, as well as GSS/SSPI
authentication as separate message formats. Even though they're all 'p'
messages, and the exact format depends on the context, describing them as
separate message formats makes the documentation more clear.

Reviewed by Michael Paquier and Álvaro Hernández Tortosa.

Discussion: https://www.postgresql.org/message-id/CAB7nPqS-aFg0iM3AQOJwKDv_0WkAedRjs1W2X8EixSz+sKBXCQ@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4f3b87ab780b95c2cc8a591259baefaff4852037

Modified Files
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doc/src/sgml/protocol.sgml | 418 ++++++++++++++++++++++++++++++++++++++---
src/backend/libpq/auth-scram.c | 27 ++-
src/backend/libpq/auth.c | 68 ++++++-
src/include/libpq/pqcomm.h | 5 +-
src/interfaces/libpq/fe-auth.c | 158 +++++++++++-----
5 files changed, 588 insertions(+), 88 deletions(-)