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/nssg.h

@@ -42,20 +42,6 @@ int Curl_nss_close_all(struct SessionHandle *data);
 int Curl_nss_init(void);
 void Curl_nss_cleanup(void);
 
-/* for documentation see Curl_ssl_send() in sslgen.h */
-int Curl_nss_send(struct connectdata *conn,
-                  int sockindex,
-                  const void *mem,
-                  size_t len,
-                  int *curlcode);
-
-/* for documentation see Curl_ssl_recv() in sslgen.h */
-ssize_t Curl_nss_recv(struct connectdata *conn, /* connection data */
-                      int num,                  /* socketindex */
-                      char *buf,                /* store read data here */
-                      size_t buffersize,        /* max amount to read */
-                      int *curlcode);
-
 size_t Curl_nss_version(char *buffer, size_t size);
 int Curl_nss_check_cxn(struct connectdata *cxn);
 int Curl_nss_seed(struct SessionHandle *data);
@@ -74,8 +60,6 @@ int Curl_nss_seed(struct SessionHandle *data);
 #define curlssl_set_engine(x,y) (x=x, y=y, CURLE_FAILED_INIT)
 #define curlssl_set_engine_default(x) (x=x, CURLE_FAILED_INIT)
 #define curlssl_engines_list(x) (x=x, (struct curl_slist *)NULL)
-#define curlssl_send Curl_nss_send
-#define curlssl_recv Curl_nss_recv
 #define curlssl_version Curl_nss_version
 #define curlssl_check_cxn(x) Curl_nss_check_cxn(x)
 #define curlssl_data_pending(x,y) (x=x, y=y, 0)