Remove `BO_FNAME_SIZE_{,NO}CRYPT`

The `block0` struct can't be changed because of backwards compatibility (see
the comments at the definition). Because this definition can't change, but
more information is needed, vim uses the pretty nasty hack of storing file
encoding information in the last bytes of `b0_fname`. Vim also reserves
space for the encryption seed in this array. Since this is no longer
needed, it is always enough to just reserve the last 2 bytes.

Closes #836