Increase array and string max size

[luser-dr00g]

Currently, xpost does not allocate an ent number for zero-sized array. And it uses a 16bit field for sizes of array and string objects. The underlying memory system can handle 32bit sizes.

So, we can interpret size=0 to be size=0x10000 iff the ent is not 0. This increases the maximum size of strings and arrays and dicts to 65536.