tighten Storable's recognition of tied SVs

Since commit ff44333e5a9d9dca5272bb166df463607ebd3020, being RMAGICAL
and having tie magic is not sufficient to recognise an SV as tied.
When magic is turned off for mg_set(), the RMAGICAL flag is now left on,
so that vstrings will be recognised as such.  So Storable needs to check
whether the tie magic it sees is actually in effect, by also looking at
the GMAGICAL and SMAGICAL flags.