KNOWN_BUGS: Use https://curl.haxx.se URL for github based issues

docs/KNOWN_BUGS

@@ -6,17 +6,17 @@ may have been fixed since this was written!
 94. Replies to IMAP custom requests other than LIST do not appear in the body.
   If you send a custom request such as FETCH the server's reply will be parsed
   as if it is a reply to a LIST.
-  https://github.com/curl/curl/issues/536
+  https://curl.haxx.se/bug/?i=536
 
 93. It is not possible to pass a 64-bit value using CURLFORM_CONTENTLEN with
   CURLFORM_ARRAY, when compiled on 32-bit platforms that support 64-bit
   integers. This is because the underlying structure 'curl_forms' uses a dual
   purpose char* for storing these values in via casting. For more information
   see the now closed related issue:
-  https://github.com/curl/curl/issues/608
+  https://curl.haxx.se/bug/?i=608
 
 91. "curl_easy_perform hangs with imap and PolarSSL"
-  https://github.com/curl/curl/issues/334
+  https://curl.haxx.se/bug/?i=334
 
 90. IMAP "SEARCH ALL" truncates output on large boxes. "A quick search of the
   code reveals that pingpong.c contains some truncation code, at line 408,
@@ -44,7 +44,7 @@ may have been fixed since this was written!
   CURLINFO_STARTTRANSFER_TIME is wrong. While using POST
   CURLINFO_STARTTRANSFER_TIME minus CURLINFO_PRETRANSFER_TIME is near to zero
   every time.
-  https://github.com/curl/curl/issues/218
+  https://curl.haxx.se/bug/?i=218
   https://curl.haxx.se/bug/view.cgi?id=1213
 
 84. CURLINFO_SSL_VERIFYRESULT is only implemented for the OpenSSL and NSS