Remove return value of AEAD_Mode::start as it doesn't seem necessary
and is an extra complication for callers.

Replace the get_aead in lookup.h returning a Filter with one in aead.h
returning AEAD_Mode.
Make SHA-256 the default instead of SHA-1 in passhash9 - it's been
supported since 1.8.10, so shouldn't be any problems there.

Add support for SHA-384 and SHA-512.

Check for work factors over 512 and reject for now as too large.
Change the default PKCS #8 PBKDF runtime from 200 to 300 milliseconds.

Round PBKDF1 and PBKDF2 time-based iterations to multiples of 10000
instead of 8192.

Update the password hashing docs a bit.