pgsql: Correct constness of a few variables.

Correct constness of a few variables.

This allows the compiler / linker to mark affected pages as read-only.

There's other cases, but they're a bit more invasive, and should go
through some review. These are easy.

They were found with
objdump -j .data -t src/backend/postgres|awk '{print $4, $5, $6}'|sort -r|less

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/62649bad831fb69b5d9644470acc413a35cddea6

Modified Files
--------------
src/backend/commands/event_trigger.c | 4 ++--
src/backend/foreign/foreign.c | 6 +++---
src/backend/libpq/pqcomm.c | 4 ++--
src/backend/libpq/pqmq.c | 2 +-
src/backend/replication/basebackup.c | 4 ++--
src/backend/storage/lmgr/generate-lwlocknames.pl | 2 +-
src/backend/utils/adt/cash.c | 4 ++--
src/include/libpq/libpq.h | 2 +-
src/include/storage/lwlock.h | 2 +-
9 files changed, 15 insertions(+), 15 deletions(-)