small cleanup: unify scanstr() functions

Hi,

Looking at the guc file code, GUC_scanstr() is almost the same as the
exported function scanstr(), except the latter requires finicky extra
coding around double quotes both in its callers and while creating the
input.

In the attached, the GUC_scanstr() function body is moved to scansup.c
to replace scanstr(), but with a different order of if-statements to
make the diff smaller. Since we have control over what goes in the BKI
file, we can use single-quoted escape strings there, allowing removal
of special case code for double quotes.

--
John Naylor                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services