Copy keys for aassign in lvalue sub

Copy keys for aassign in lvalue sub

Checking LVRET (which pp_aassign does, as of a few commits ago)
has no effect if OPpMAYBE_LVSUB is not set on the op.  This com-
mit changes op.c:op_lvalue_flags to set this flag on aassign ops.

This makes sub:lvalue{%h=($x,$x)} behave correctly if the return
values of the sub are assigned to ($x is unmodfied).