pgsql: Avoid premature de-doubling of quote marks in ECPG strings.

Avoid premature de-doubling of quote marks in ECPG strings.

If you write the literal 'abc''def' in an EXEC SQL command, that will
come out the other end as 'abc'def', triggering a syntax error in the
backend.  Likewise, "abc""def" is reduced to "abc"def" which is wrong
syntax for a quoted identifier.

The cause is that the lexer thinks it should emit just one quote
mark, whereas what it really should do is keep the string as-is.

Add some docs and test cases, too.

Although this seems clearly a bug, I fear users wouldn't appreciate
changing it in minor releases.  Some may well be working around it
by applying an extra doubling of affected quotes, as for example
sql/dyntest.pgc has been doing.

Per investigation of a report from 1250kv, although this isn't
exactly what he/she was on about.

Discussion: https://postgr.es/m/673825.1603223178@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3dfb1942d9b8748b93094a430289e2f7f3b3ae0d

Modified Files
--------------
doc/src/sgml/ecpg.sgml                                  | 17 ++++++++++++++---
src/interfaces/ecpg/preproc/pgc.l                       |  9 +++------
src/interfaces/ecpg/test/expected/preproc-strings.c     |  2 +-
.../ecpg/test/expected/preproc-strings.stderr           |  8 ++++----
.../ecpg/test/expected/preproc-strings.stdout           |  2 +-
src/interfaces/ecpg/test/preproc/strings.pgc            |  8 ++++----
src/interfaces/ecpg/test/sql/dyntest.pgc                |  2 +-
7 files changed, 28 insertions(+), 20 deletions(-)