IGNOREEOF and POSIXLY_CORRECT don't ignore invisible vars

[Grisha Levit]

The manual references these values being set, not just declared, so maybe
should check for invisible_p?

Noticeable for example because this works:

$ set +o posix; f() { local POSIXLY_CORRECT=; shopt -p -o posix; }; f;
shopt -p -o posix
set -o posix
set +o posix

But the opposite does not:

$ set -o posix; f() { local POSIXLY_CORRECT; shopt -p -o posix; }; f; shopt
-p -o posix
set -o posix
set -o posix