Refactor IO flags and EOF handling, with the goal of handle type unification.

Also removes the deprecated Parrot_io_socket_handle() as well as Parrot_io_get_flags()
and Parrot_io_set_flags(), which were never marked PARROT_EXPORT, and
Parrot_io_socket_initialize() wich was.

Adds the functions Parrot_io_raise_flag() and Parrot_io_clear_flag() - which delegate to
the apply_flag() and remove_flag() IO vtables - and Parrot_io_clear_eof().

All tests except t/dynpmc/gziphandle.t pass, NQP and Rakudo still build.