pgsql: Fix t_isspace(), etc., when datlocprovider=i and datctype=C.

Fix t_isspace(), etc., when datlocprovider=i and datctype=C.

Check whether the datctype is C to determine whether t_isspace() and
related functions use isspace() or iswspace().

Previously, t_isspace() checked whether the database default collation
was C; which is incorrect when the default collation uses the ICU
provider.

Discussion: https://postgr.es/m/79e4354d9eccfdb00483146a6b9f6295202e7890.camel@j-davis.com
Reviewed-by: Peter Eisentraut
Backpatch-through: 15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f413941f41d370a7893caa3e6ed384b89a0577fd

Modified Files
--------------
contrib/unaccent/expected/unaccent.out | 9 ---------
contrib/unaccent/expected/unaccent_1.out | 8 --------
contrib/unaccent/sql/unaccent.sql | 11 -----------
src/backend/tsearch/ts_locale.c | 18 ++++++------------
src/backend/tsearch/wparser_def.c | 3 +--
src/backend/utils/adt/pg_locale.c | 3 +++
src/backend/utils/init/postinit.c | 4 ++++
src/include/utils/pg_locale.h | 2 ++
8 files changed, 16 insertions(+), 42 deletions(-)