lua: don't modify pointer type in msgpack.decode*

msgpackffi.decode_unchecked([const] char *) returns two values: a
decoded result and a new pointer within passed buffer. After #3926 a
cdata type of the returned pointer follows a type of passed buffer.

This commit modifies behaviour of msgpack module in the same way. The
following functions now returns cdata<char *> or cdata<const char *>
depending of its argument:

* msgpack.decode(cdata<[const] char *>, number)
* msgpack.decode_unchecked(cdata<[const] char *>)
* msgpack.decode_array_header(cdata<[const] char *>, number)
* msgpack.decode_map_header(cdata<[const] char *>, number)

Follows up #3926.