[perl #77684] Restore the 5.10/12 behaviour of open $fh, ">", \$glob_copy

[perl #77684] Restore the 5.10/12 behaviour of open $fh, ">", \$glob_copy

This restores the perl 5.10/12 behaviour, making open treat \$foo as a
scalar reference if it is a glob copy (SvFAKE).

It also fixes an existing assertion failure that the test now trig-
gers. PerlIOScalar_pushed was not downgrading the sv before set-
ting SvCUR.