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

secure/lib/libcrypt/crypt-blowfish.c

@@ -167,6 +167,9 @@ crypt_blowfish(const char *key, const char *salt)
 			 switch (salt[1]) {
 			 case 'a':	/* 'ab' should not yield the same as 'abab' */
 			 case 'b':	/* cap input length at 72 bytes */
+			 case 'y':	/* same as 'b', for compatibility
+					 * with openwall crypt_blowfish
+					 */
 				 minr = salt[1];
 				 salt++;
 				 break;