Magic flags harmonization.

Magic flags harmonization.

In restore_magic(), which is called after any magic processing, all of
the public OK flags have been shifted into the private OK flags.  Thus
the lack of an appropriate public OK flags was used to trigger both get
magic and required conversions.  This scheme did not cover ROK, however,
so all properly written code had to make sure mg_get was called the right
number of times anyway.  Meanwhile the private OK flags gained a second
purpose of marking converted but non-authoritative values (e.g. the IV
conversion of an NV), and the inadequate flag shift mechanic broke this
in some cases.

This patch removes the shift mechanic for magic flags, thus exposing (and
fixing) some improper usage of magic SVs in which mg_get() was not called
correctly.  It also has the side effect of making magic get functions
specifically set their SVs to undef if that is desired, as the new behavior
of empty get functions is to leave the value unchanged.  This is a feature,
as now get magic that does not modify its value, e.g. tainting, does not
have to be special cased.

The changes to cpan/ here are only temporary, for development only, to
keep blead working until upstream applies them (or something like them).

Thanks to Rik and Father C for review input.