postgres_fdw: Tighten up allowed values for batch_size, fetch_size options.

postgres_fdw: Tighten up allowed values for batch_size, fetch_size options.

Previously the values such as '100$%$#$#', '9,223,372,' were accepted and
treated as valid integers for postgres_fdw options batch_size and fetch_size.
Whereas this is not the case with fdw_startup_cost and fdw_tuple_cost options
for which an error is thrown. This was because endptr was not used
while converting strings to integers using strtol.

This commit changes the logic so that it uses parse_int function
instead of strtol as it serves the purpose by returning false in case
if it is unable to convert the string to integer. Note that
this function also rounds off the values such as '100.456' to 100 and
'100.567' or '100.678' to 101.

While on this, use parse_real for fdw_startup_cost and fdw_tuple_cost options.

Since parse_int and parse_real are being used for reloptions and GUCs,
it is more appropriate to use in postgres_fdw rather than using strtol
and strtod directly.

Back-patch to v14.

Author: Bharath Rupireddy
Reviewed-by: Ashutosh Bapat, Tom Lane, Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACVMO6wY5Pc4oe1OCgUOAtdjHuFsBDw8R5uoYR86eWFQDA@mail.gmail.com