pgsql: Use guc.c's parse_int() instead of pg_atoi() to parse fillfactor

Log Message:
-----------
Use guc.c's parse_int() instead of pg_atoi() to parse fillfactor in
default_reloptions(). The previous coding was really a bug because pg_atoi()
will always throw elog on bad input data, whereas default_reloptions is not
supposed to complain about bad input unless its validate parameter is true.
Right now you could only expose the problem by hand-modifying
pg_class.reloptions into an invalid state, so it doesn't seem worth
back-patching; but we should get it right in HEAD because there might be other
situations in future. Noted while studying GIN fast-update patch.

Modified Files:
--------------
pgsql/src/backend/access/common:
reloptions.c (r1.10 -> r1.11)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/reloptions.c?r1=1.10&r2=1.11)
pgsql/src/backend/utils/misc:
guc.c (r1.464 -> r1.465)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.464&r2=1.465)
pgsql/src/include/utils:
guc.h (r1.97 -> r1.98)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc.h?r1=1.97&r2=1.98)