Integrate perlio:

[ 14335]
Split out core of sv_magic() into sv_magicext().
sv_magic provides the extra restictions (no READONLY, only
one of each type, canned set of vtables), and sv_magicext()
does the actual data twiddling.
Also enhances semantics of ->mg_ptr setting via name/namlen
to allow either an uncopied ptr (namlen == 0), or a Newz()ed
scratch area (namlen > 0 && name == NULL).
sv_magicext also returns the MAGIC * it added.
sv_magicext is intended mainly for PERL_MAGIC_ext (~) magic.
To come sv_unmagicext() - which will remove just one magic
of particular type, and additionaly match against ->mg_ptr,
or the MAGIC * (need to experiment as to which is more natural).

p4raw-link: @14335 on //depot/perlio: 92110913508b9944d111285d9488f2f7b604919c

p4raw-id: //depot/perl@14346

diff --cc embed.fnc
Simple merge

diff --cc util.c
Simple merge