pgsql: Fix some cases of indirectly casting away const.

Fix some cases of indirectly casting away const.

Newest versions of gcc+glibc are able to detect cases where code
implicitly casts away const by assigning the result of strchr() or
a similar function applied to a "const char *" value to a target
variable that's just "char *". This of course creates a hazard of
not getting a compiler warning about scribbling on a string one was
not supposed to, so fixing up such cases is good.

This patch fixes a dozen or so places where we were doing that.
Most are trivial additions of "const" to the target variable,
since no actually-hazardous change was occurring.

Thanks to Bertrand Drouvot for finding a couple more spots than
I had.

This commit back-patches relevant portions of 8f1791c61 and
9f7565c6c into supported branches. However, there are two
places in ecpg (in v18 only) where a proper fix is more
complicated than seems appropriate for a back-patch. I opted
to silence those two warnings by adding casts.

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/1324889.1764886170@sss.pgh.pa.us
Discussion: https://postgr.es/m/3988414.1771950285@sss.pgh.pa.us
Backpatch-through: 14-18

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0969bfd0db869a31e341ad17bbef32532f6315fc

Modified Files
--------------
src/backend/catalog/pg_type.c | 2 +-
src/backend/tsearch/spell.c | 2 +-
src/backend/utils/adt/formatting.c | 5 +++--
src/backend/utils/adt/pg_locale.c | 2 +-
src/backend/utils/adt/xid8funcs.c | 2 +-
src/bin/pg_waldump/pg_waldump.c | 2 +-
src/bin/pgbench/pgbench.c | 2 +-
src/common/compression.c | 2 +-
src/interfaces/ecpg/pgtypeslib/datetime.c | 4 ++--
src/port/getopt.c | 2 +-
src/port/getopt_long.c | 2 +-
src/port/win32setlocale.c | 8 ++++----
src/test/regress/pg_regress.c | 2 +-
src/timezone/zic.c | 2 +-
14 files changed, 20 insertions(+), 19 deletions(-)