Enable standard_conforming_strings to be turned on.

doc/src/sgml/config.sgml

src/backend/parser/scan.l

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/bin/psql/common.c

src/bin/psql/common.h

src/bin/psql/psqlscan.l

src/include/utils/guc.h

src/interfaces/ecpg/preproc/pgc.l

src/test/regress/expected/strings.out

src/test/regress/sql/strings.sql