pgsql: Use PqMsg_* macros in more places.

Use PqMsg_* macros in more places.

Commit f4b54e1ed9, which introduced macros for protocol characters,
missed updating a few places.  It also did not introduce macros for
messages sent from parallel workers to their leader processes.
This commit adds a new section in protocol.h for those.

Author: Aleksander Alekseev
Discussion: https://postgr.es/m/CAJ7c6TNTd09AZq8tGaHS3LDyH_CCnpv0oOz2wN1dGe8zekxrdQ%40mail.gmail.com
Backpatch-through: 17

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/925479b8d83c2fc819e05bf67335255c13d8e8de

Modified Files
--------------
src/backend/access/common/printtup.c          | 5 +++--
src/backend/commands/explain.c                | 3 ++-
src/backend/replication/walsender.c           | 2 +-
src/backend/tcop/postgres.c                   | 3 +--
src/backend/utils/activity/backend_progress.c | 4 ++--
src/include/libpq/protocol.h                  | 4 ++++
6 files changed, 13 insertions(+), 8 deletions(-)