sendrecv: split the I/O handling into private handler

Howard Chu brought the bulk work of this patch that properly
moves out the sending and recving of data to the parts of the
code that are properly responsible for the various ways of doing
so.

Daniel Stenberg assisted with polishing a few bits and fixed some
minor flaws in the original patch.

Another upside of this patch is that we now abuse CURLcodes less
with the "magic" -1 return codes and instead use CURLE_AGAIN more
consistently.

lib/sslgen.c

@@ -410,25 +410,6 @@ struct curl_slist *Curl_ssl_engines_list(struct SessionHandle *data)
   return curlssl_engines_list(data);
 }
 
-ssize_t Curl_ssl_send(struct connectdata *conn,
-                      int sockindex,
-                      const void *mem,
-                      size_t len,
-                      int *curlcode)
-{
-  return curlssl_send(conn, sockindex, mem, len, curlcode);
-}
-
-ssize_t Curl_ssl_recv(struct connectdata *conn,
-                      int sockindex,
-                      char *mem,
-                      size_t len,
-                      int *curlcode)
-{
-  return curlssl_recv(conn, sockindex, mem, len, curlcode);
-}
-
-
 /*
  * This sets up a session ID cache to the specified size. Make sure this code
  * is agnostic to what underlying SSL technology we use.