Add compatibility with $2y$ bcrypt hashes

crypt_blowfish and many implementations based on it (Apache, PHP, PostgreSQL) implemented $2y$ before OpenBSD went with $2b$. This changes marks them as equivalent.

http://www.openwall.com/lists/announce/2011/07/17/1

This change is required for applications that use the base crypt() implementation (including nginx) to be able to validate $2y$ hashes

Reviewed by: eadler
Approved by: delphij
MFC after: 1 week
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D2742