In PL_magic_data flag whether magic can be added to a readonly value.

In PL_magic_data flag whether magic can be added to a readonly value.

Use this to simplify the logic in Perl_sv_magic().

This introduces a small change of behaviour for error cases involving unknown
magic types. Previously, if Perl_sv_magic() was passed a magic type unknown to
it, it would

1: Croak "Modification of a read-only value attempted" if read only
2: Return without error if the SV happened to already have this magic
3: otherwise croak "Don't know how to handle magic of type \\%o"

Now it will always croak "Don't know how to handle magic of type \\%o", even
on read only values, or SVs which already have the unknown magic type.