Add the CRC32 compensation attack detector that all other SSH

clients have had for ages and I forgot about. Of course I've got the
version with the buffer overflow fixed!

[originally from svn r1535]

sshcrc.c

@@ -211,9 +211,8 @@ int main(void)
 }
 #endif
 
-unsigned long crc32(const void *buf, size_t len)
+unsigned long crc32_update(unsigned long crcword, const void *buf, size_t len)
 {
-    unsigned long crcword = 0L;
     const unsigned char *p = (const unsigned char *) buf;
     while (len--) {
 	unsigned long newbyte = *p++;
@@ -222,3 +221,8 @@ unsigned long crc32(const void *buf, size_t len)
     }
     return crcword;
 }
+
+unsigned long crc32(const void *buf, size_t len)
+{
+    return crc32_update(0L, buf, len);
+}