call pp_glob() even when its being skipped

call pp_glob() even when its being skipped

Currently when an external Perl glob function is used (which is most of
the time), the OP_GLOB op is removed and replaced with the pair:
    GV("CORE::GLOBAL::glob"), ENTERSUB.

This commit re-adds the OP_GLOB to the op tree, but with OPf_SPECIAL set;
and pp_glob() is updated to just return if OPf_SPECIAL is set.

Thus there's no change in outward functionality with this commit. However,
by always calling pp_glob(), it will allow us (in the next commit) to
handle iterator overloading consistently, regardless of whether the
internal globbing function is used or not.