limit sha512 rounds to similar runtime to sha256 limit

these limits could definitely use review, but for now, i feel
consistency and erring on the side of preventing servers from getting
bogged down by excessively-slow user-provided settings (think
.htpasswd) are the best policy. blowfish should be updated to match.

src/misc/crypt_sha512.c

@@ -193,7 +193,7 @@ static char *to64(char *s, unsigned int u, int n)
 #define SALT_MAX 16
 #define ROUNDS_DEFAULT 5000
 #define ROUNDS_MIN 1000
-#define ROUNDS_MAX 50000
+#define ROUNDS_MAX 20000
 
 /* hash n bytes of the repeated md message digest */
 static void hashmd(struct sha512 *s, unsigned int n, const void *md)