pgsql: Stop using spelling "nonexistant".

Stop using spelling "nonexistant".

The documentation used "nonexistent" exclusively, and the source tree
used it three times as often as "nonexistant".

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f2c71cb71fe186dcf8ebec967f98d1c7c4777943

Modified Files
--------------
contrib/test_decoding/expected/ddl.out | 4 +-
contrib/test_decoding/sql/ddl.sql | 2 +-
src/bin/pg_dump/t/002_pg_dump.pl | 8 ++--
src/interfaces/ecpg/test/connect/test1.pgc | 2 +-
src/interfaces/ecpg/test/connect/test5.pgc | 2 +-
.../test/expected/connect-test1-minGW32.stderr | 8 ++--
src/interfaces/ecpg/test/expected/connect-test1.c | 2 +-
.../ecpg/test/expected/connect-test1.stderr | 8 ++--
src/interfaces/ecpg/test/expected/connect-test5.c | 2 +-
.../ecpg/test/expected/connect-test5.stderr | 2 +-
.../ecpg/test/expected/preproc-whenever.c | 10 ++---
.../ecpg/test/expected/preproc-whenever.stderr | 46 +++++++++++-----------
src/interfaces/ecpg/test/preproc/whenever.pgc | 10 ++---
src/test/regress/expected/alter_table.out | 6 +--
src/test/regress/expected/create_am.out | 4 +-
src/test/regress/expected/rowtypes.out | 6 +--
src/test/regress/expected/stats_ext.out | 4 +-
src/test/regress/sql/alter_table.sql | 4 +-
src/test/regress/sql/create_am.sql | 4 +-
src/test/regress/sql/rowtypes.sql | 2 +-
src/test/regress/sql/stats_ext.sql | 2 +-
21 files changed, 69 insertions(+), 69 deletions(-)