pgsql: Prevent duplicate escape-string warnings when using pg_stat_stat

Prevent duplicate escape-string warnings when using pg_stat_statements.

contrib/pg_stat_statements will sometimes run the core lexer a second time
on submitted statements. Formerly, if you had standard_conforming_strings
turned off, this led to sometimes getting two copies of any warnings
enabled by escape_string_warning. While this is probably no longer a big
deal in the field, it's a pain for regression testing.

To fix, change the lexer so it doesn't consult the escape_string_warning
GUC variable directly, but looks at a copy in the core_yy_extra_type state
struct. Then, pg_stat_statements can change that copy to disable warnings
while it's redoing the lexing.

It seemed like a good idea to make this happen for all three of the GUCs
consulted by the lexer, not just escape_string_warning. There's not an
immediate use-case for callers to adjust the other two AFAIK, but making
it possible is easy enough and seems like good future-proofing.

Arguably this is a bug fix, but there doesn't seem to be enough interest to
justify a back-patch. We'd not be able to back-patch exactly as-is anyway,
for fear of breaking ABI compatibility of the struct. (We could perhaps
back-patch the addition of only escape_string_warning by adding it at the
end of the struct, where there's currently alignment padding space.)

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/eb213acfe2a51ab3dc6d9bbe02d5d8a903366779

Modified Files
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contrib/pg_stat_statements/pg_stat_statements.c | 3 +++
src/backend/parser/scan.l | 18 +++++++++++-------
src/include/parser/scanner.h | 10 ++++++++++
3 files changed, 24 insertions(+), 7 deletions(-)