pgsql: Avoid splitting C string literals with \-newline

Avoid splitting C string literals with \-newline

Using \ is unnecessary and ugly, so remove that.  While at it, stitch
the literals back into a single line: we've long discouraged splitting
error message literals even when they go past the 80 chars line limit,
to improve greppability.

Leave contrib/tablefunc alone.

Discussion: https://postgr.es/m/20191223195156.GA12271@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c4dcd9144ba64946c9f9466748bdb2c51719c8a3

Modified Files
--------------
contrib/dblink/dblink.c                         | 15 +++++----------
contrib/pg_stat_statements/pg_stat_statements.c |  3 +--
contrib/tablefunc/tablefunc.c                   | 12 ++++--------
src/backend/access/transam/xlog.c               |  4 +---
src/backend/access/transam/xlogfuncs.c          |  3 +--
src/backend/commands/extension.c                |  9 +++------
src/backend/commands/prepare.c                  |  3 +--
src/backend/executor/execExprInterp.c           |  4 ++--
src/backend/libpq/hba.c                         |  3 +--
src/backend/replication/logical/launcher.c      |  3 +--
src/backend/replication/slotfuncs.c             |  3 +--
src/backend/replication/walsender.c             |  3 +--
src/backend/utils/adt/pgstatfuncs.c             |  6 ++----
src/backend/utils/misc/guc.c                    |  3 +--
src/backend/utils/mmgr/portalmem.c              |  3 +--
15 files changed, 26 insertions(+), 51 deletions(-)