fix NODEFAULT_SHAREKEYS

[rurban]

When NODEFAULT_SHAREKEYS is defined, sv_clear of a IsCOW/LEN=0 PV fails to get the he before the hek, which is shared with the pv.

With default SHAREKEYS every single key is stored twice. Once in strtab, and once in the hash. This seems to be a massive overhead. But I cannot test the overhead with this failure. (=> 20-30% faster)

This is an upstream bug, typical bitrot. perl5 simply assumes that every single non-strtab hash uses shared keys, ignoring the HVhek_UNSHARED bit somewhere. Or most likely forgetting to copy this bit.
NODEFAULT_SHAREKEYS was previously in newHV() in hv.c, added with fde52b5 in perl 5.003_01

repro: Configure with -DDEBUGGING -Accflags=-DNODEFAULT_SHAREKEYS;
make miniperl; ./miniperl -Ilib configpm && make uni.data and see the unshare_hek assert.
This even happens with 5.10.0, before new IsCOW times, but works with 5.8.9. So it got broken during the 5.9 cycle, probably with the old COW.

[rurban]

upstream bug bisected with git bisect start perl-5.10.0 perl-5.9.0;
git bisect run ../bisect-miniperl.sh

../bisect-miniperl.sh

#!/bin/sh
# exit 125 marks it as untestable, == bisect skip
# exit >128 stops the bisect run immediately.

reset=0
make -s -j4 miniperl 2>/dev/null || \
    (git reset --hard;
     git clean -dxf;
     export reset=1;
     perl -MDevel::PatchPerl -e'Devel::PatchPerl::patch_source'; \
     ./Configure -des -Dusedevel -DEBUGGING \
                 -Accflags=-DNODEFAULT_SHAREKEYS >/dev/null 2>/dev/null && \
     make -s -j4 miniperl 2>/dev/null) || exit 1

./miniperl -Ilib configpm && make -s uni.data
err=$?
git reset --hard
if [ $reset -ne 0 ]; then
    git clean -dxf
    unset reset
fi
if [ $err -ne 0 ]; then exit 1; fi

to

commit a51caccf222faf8588ed061240391957b9a0e70d
Author: Nicholas Clark <nick@ccl4.org>
Date:   Tue Apr 11 16:17:13 2006 +0000

    Stop Perl_newSVpvn_share() potentially leaking the return result from
    bytes_from_utf8().

    p4raw-id: //depot/perl@27766

[rurban]

benchmarks: simple bench.pl without many hashes: on linux 27% faster with NODEFAULT_SHAREKEYS.
on darwin the fluctuation and loadavg is too high to be meaningful. barely any measurable differences.