pgsql: Introduce macros for protocol characters.

Introduce macros for protocol characters.

This commit introduces descriptively-named macros for the
identifiers used in wire protocol messages. These new macros are
placed in a new header file so that they can be easily used by
third-party code.

Author: Dave Cramer
Reviewed-by: Alvaro Herrera, Tatsuo Ishii, Peter Smith, Robert Haas, Tom Lane, Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/CADK3HHKbBmK-PKf1bPNFoMC%2BoBt%2BpD9PH8h5nvmBQskEHm-Ehw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f4b54e1ed9853ab9aff524494866823f951b1e7f

Modified Files
--------------
src/backend/access/common/printsimple.c | 5 +-
src/backend/access/transam/parallel.c | 14 +++---
src/backend/backup/basebackup_copy.c | 16 +++----
src/backend/commands/async.c | 2 +-
src/backend/commands/copyfromparse.c | 22 ++++-----
src/backend/commands/copyto.c | 6 +--
src/backend/libpq/auth-sasl.c | 2 +-
src/backend/libpq/auth.c | 8 ++--
src/backend/postmaster/postmaster.c | 2 +-
src/backend/replication/walsender.c | 18 +++----
src/backend/tcop/dest.c | 8 ++--
src/backend/tcop/fastpath.c | 2 +-
src/backend/tcop/postgres.c | 68 +++++++++++++-------------
src/backend/utils/error/elog.c | 5 +-
src/backend/utils/misc/guc.c | 2 +-
src/include/Makefile | 3 +-
src/include/libpq/pqcomm.h | 23 +++------
src/include/libpq/protocol.h | 85 +++++++++++++++++++++++++++++++++
src/include/meson.build | 1 +
src/interfaces/libpq/fe-auth.c | 2 +-
src/interfaces/libpq/fe-connect.c | 19 +++++---
src/interfaces/libpq/fe-exec.c | 54 ++++++++++-----------
src/interfaces/libpq/fe-protocol3.c | 70 ++++++++++++++-------------
src/interfaces/libpq/fe-trace.c | 70 +++++++++++++++------------
src/tools/msvc/Install.pm | 2 +
25 files changed, 305 insertions(+), 204 deletions(-)