Accept older blowfish hashes.

Both `2a` and `2x` are valid types of blowfish hashes, that while being
older should be accepted.

Backport 00c94bd582b83f8b92228b750aea0e8816a4ea89 from 3.x to 2.5.x,
I see this as a bug fix as it fixes incompatibilities with hashes
created by hash_password().

Refs #3575