[perl #128187] Forbid sub :lvalue{keys} in aassign

[perl #128187] Forbid sub :lvalue{keys} in aassign

This commit makes perl die when keys(%hash) is returned from an lvalue
sub and the lvalue sub call is assigned to in list assignment:

    sub foo : lvalue { keys(%INC) }
    (foo) = 3; # death

This prevents an assignment that is completely useless and probably a
mistake, and it makes the lvalue-sub use of keys behave the same way
as (keys(%INC)) = 3.