postgres_fdw: Mark statistics import helpers as static

postgres_fdw: Mark statistics import helpers as static

The set_*_arg helper functions in postgres_fdw.c are declared
static, but their definitions omitted the static keyword. Add it to
make their file-local scope explicit and keep the declarations and
definitions consistent.

Also fix a couple of nearby comment typos.

This is a followup to commit 54cd6fc8317.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwGjcQ4SwHMUQ9P8UYQ7iLKL1QE3uLSdONToQ1MrzpUUoQ@mail.gmail.com
Backpatch-through: 19