pgcrypto support for bcrypt $2b$ hashes

Hello,

I've recently been working with a database containing bcrypt hashes generated by a 3rd-party which use the $2b$ prefix.
Thisprefix was introduced in 2014 and has since been recognised by a number of bcrypt implementations. [1][2][3][4] 

At the moment, pgcrypto’s `crypt` doesn’t recognise this prefix. However, simply `replace`ing the prefix with $2a$
allowscrypt to validate the hashes. This patch simply adds recognition for the prefix and treats the hash identically
tothe $2a$ hashes. 

Is this a reasonable change to pgcrypto? I note that the last upstream change brought into crypt-blowfish.c was in
2011,predating this prefix. [5] Are there deeper concerns or other upstream changes that need to be addressed alongside
this?Is there a better approach to this?  

At the moment, the $2x$ variant is supported but not mentioned in the docs, so I haven’t included any documentation
updates.

Thanks,

Daniel


[1]: https://marc.info/?l=openbsd-misc&m=139320023202696
[2]: https://www.openwall.com/lists/announce/2014/08/31/1
[3]:
https://github.com/kelektiv/node.bcrypt.js/pull/549/files#diff-c55280c5e4da52b0f86244d3b95c5ae0abf2fcd121a071dba1363540875b32bc
[4]: https://github.com/bcrypt-ruby/bcrypt-ruby/commit/d19ea481618420922b533a8b0ed049109404cb13
[5]: https://github.com/postgres/postgres/commit/ca59dfa6f727fe3bf3a01904ec30e87f7fa5a67e