pgsql: Refactor SASL exchange to return tri-state status

Refactor SASL exchange to return tri-state status

The SASL exchange callback returned state in to output variables:
done and success. This refactors that logic by introducing a new
return variable of type SASLStatus which makes the code easier to
read and understand, and prepares for future SASL exchanges which
operate asynchronously.

This was extracted from a larger patchset to introduce OAuthBearer
authentication and authorization.

Author: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/d1b467a78e0e36ed85a09adf979d04cf124a9d4b.camel@vmware.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/24178e235ea56aca9233e640dd7ff9b17e858b07

Modified Files
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src/interfaces/libpq/fe-auth-sasl.h | 31 +++++++++-----
src/interfaces/libpq/fe-auth-scram.c | 78 +++++++++++++++++-------------------
src/interfaces/libpq/fe-auth.c | 28 ++++++-------
src/tools/pgindent/typedefs.list | 1 +
4 files changed, 71 insertions(+), 67 deletions(-)