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/sendf.h

@@ -55,15 +55,20 @@ CURLcode Curl_client_write(struct connectdata *conn, int type, char *ptr,
                            size_t len);
 
 /* internal read-function, does plain socket only */
-int Curl_read_plain(curl_socket_t sockfd,
-                    char *buf,
-                    size_t bytesfromsocket,
-                    ssize_t *n);
+CURLcode Curl_read_plain(curl_socket_t sockfd,
+                         char *buf,
+                         size_t bytesfromsocket,
+                         ssize_t *n);
+
+ssize_t Curl_recv_plain(struct connectdata *conn, int num, char *buf,
+                        size_t len, CURLcode *code);
+ssize_t Curl_send_plain(struct connectdata *conn, int num,
+                        const void *mem, size_t len, CURLcode *code);
 
 /* internal read-function, does plain socket, SSL and krb4 */
-int Curl_read(struct connectdata *conn, curl_socket_t sockfd,
-              char *buf, size_t buffersize,
-              ssize_t *n);
+CURLcode Curl_read(struct connectdata *conn, curl_socket_t sockfd,
+                   char *buf, size_t buffersize,
+                   ssize_t *n);
 /* internal write-function, does plain socket, SSL, SCP, SFTP and krb4 */
 CURLcode Curl_write(struct connectdata *conn,
                     curl_socket_t sockfd,