Make postgres.bki use the same literal-string syntax as postgresql.conf.

Make postgres.bki use the same literal-string syntax as postgresql.conf.

The BKI file's string quoting conventions were previously quite weird,
perhaps as a result of repurposing a function built to scan
single-quoted strings to scan double-quoted ones.  Change to use the
same rules as we use in GUC files, allowing some simplifications in
genbki.pl and initdb.c.

While at it, completely remove the backend's scanstr() function, which
was essentially a duplicate of the string dequoting code in guc-file.l.
Instead export that one (under a less generic name than it had) and let
bootscanner.l use it.  Now we can clarify that scansup.c exists only to
support the main lexer. We could alternatively have removed GUC_scanstr,
but this way seems better since the previous arrangement could mislead
a reader into thinking that scanstr() had something to do with the main
lexer's handling of string literals.  Maybe it did once, but if so it
was a long time ago.

This patch does not bump catversion, since the initially-installed
catalog contents don't change.  Note however that successful initdb
after applying this patch will require up-to-date postgres.bki as well
as postgres and initdb executables.

In passing, remove a bunch of very-long-obsolete #include's in
bootparse.y and bootscanner.l.

John Naylor

Discussion: https://postgr.es/m/CACPNZCtDpd18T0KATTmCggO2GdVC4ow86ypiq5ENff1VnauL8g@mail.gmail.com