pgsql: Convert backslash_quote guc to use enum.

Log Message:
-----------
Convert backslash_quote guc to use enum.

Modified Files:
--------------
pgsql/src/backend/utils/misc:
guc.c (r1.444 -> r1.445)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.444&r2=1.445)
pgsql/src/include/parser:
gramparse.h (r1.40 -> r1.41)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/gramparse.h?r1=1.40&r2=1.41)