Make lv keys distinguish scalar/list cx properly

Make lv keys distinguish scalar/list cx properly

keys(%h) was special in that it did not use the same code path as
other ops that distinguish between scalar and list lvalue context.
Consequently, some scalar lvalue contexts worked:

    keys %h = 3;
    ${\scalar keys %h} = 3;
    sub { $_[0] = 3 }->(scalar keys %h);
    foreach(scalar keys %h) { $_ = 3 }
    grep { $_ = 3 } scalar keys %h;
    substr keys %h, 0, = 3;

while others did not:

    keys %h .= 0;
    read FH, keys %h, 0;

Fixing other bugs in the same code paths without breaking keys (or
adding *more* exceptions) is harder to do if keys is not consistent.
So this commit allows .= and read to assign to keys, by using the same
internal code (scalar_mod_type) that determines whether %h assignment
is allowed.  The logic is reversed (since %h is list-only and keys %h
is scalar-only), so where %h is a valid lvalue keys %h is not, and
vice versa.