pgsql: Make SPI_prepare argtypes argument const

Make SPI_prepare argtypes argument const

This changes the argtypes argument of SPI_prepare(),
SPI_prepare_cursor(), SPI_cursor_open_with_args(), and
SPI_execute_with_args() from Oid *argtypes to const Oid *argtypes.
The underlying functions were already receptive to that, so this
doesn't require any significant changes beyond the function signatures
and some internal variables.

Commit 28972b6fc3dc recently introduced a case where a const had to be
cast away before calling these functions. This is fixed here.

In passing, make a very similar const addition to SPI_modifytuple().

Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Ewan Young <kdbase(dot)hack(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/86b5162f-c472-40fa-997b-0450dece1dec%40eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b1c41398e48ca7d38a46c901dc93872c968b227c

Modified Files
--------------
contrib/postgres_fdw/postgres_fdw.c | 6 +++---
doc/src/sgml/spi.sgml | 10 +++++-----
src/backend/executor/spi.c | 28 +++++++++++++++++-----------
src/backend/utils/adt/ri_triggers.c | 4 ++--
src/backend/utils/cache/plancache.c | 2 +-
src/include/executor/spi.h | 10 +++++-----
src/include/executor/spi_priv.h | 2 +-
src/include/utils/plancache.h | 2 +-
8 files changed, 35 insertions(+), 29 deletions(-)