Simplify EVP_DecryptUpdate() a bit

Simplify EVP_DecryptUpdate() a bit

This time the block size is called b and there's some awful length
fiddling with fix_len, which until recently also served as store
for the return value for do_cipher()...

If we land on a block boundary, we keep the last block decrypted and
don't count it as part of the output. So in the next call we need to
feed it back in. Feeding it back in counts as output written this time
around, so instead of remembering that we need to adjust outl, keep a
tally of the bytes written. This way we can also do some overflow and
underflow checking.

ok jsing