Casts should be checked

[benrubson]

Here is a bug report in order to think about types' check.

Going through the code, there are many places where size_t / ssize_t / off_t / int... are wrongly used and may lead to some overflows...
Some implicit and explicit casts between size_t / ssize_t / int which are also sometimes wrongly done, leading to some limits (even if they are high) which could be overflowed.

For example, read and write operations deal with as much as dataLen (size_t), and return ssize_t, which does not seem to be absolutely correct.
Another example where an int is used to go through the number of blocks (should be an off_t) :
https://github.com/vgough/encfs/blob/v1.9.2/encfs/encfsctl.cpp#L388

This is a fiddly check to do then.

As indicated in #446, I think we should keep :

• size_t for dataLen, as in read(2) / write(2) ;
• ssize_t as read & write return type, as in read(2) / write(2) ;
• off_t for sizes and offsets, as in stat(2).