No support for tarballs larger than 4GB

[gkanwar]

The implementation implicitly uses unsigned, etc which is often compiled as 32-bit words. In this case offsets will silently wrap for tarballs longer than 4GB and cause errors.

The easiest fix would be to add a separate method (mtar_add_pos(unsigned pos, unsigned offset)) which checks that the position doesn't wrap silently while doing the addition. Support for a 64-bit implementation would be nice as well :)

[katahiromz]

Use size_t type.