PR23875 :: support string map keys in foreach iteration

PR23875 :: support string map keys in foreach iteration

* bpf-translate.cxx (bpf_unparser::visit_foreach_loop): Add code to handle
and correctly allocate space for string map keys.
* bpf-interp.cxx (as_ptr): New overload yielding void * from char *,
to return cached string map keys into bpf registers.
(typedef map_int_keys): Renamed from map_keys, only handles caching integer keys.
(typedef map_str_keys): New typedef, handled caching string keys.
(struct map_keys): New struct (XXX pseudo-union) for either int or str key iteration.
(map_get_next_key): Rewrite to support both int and str key iteration,
take bpf_transport_context.
(bpf_interpret): Pass bpf_transport_context to map_get_next_key.