Rename net.{c,h} to net_sockets.{c,h}

The library/net.c and its corresponding include/mbedtls/net.h file are
renamed to library/net_sockets.c and include/mbedtls/net_sockets.h
respectively. This is to avoid naming collisions in projects which also
have files with the common name 'net'.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index c0bfd3e..1c0513d 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1105,9 +1105,10 @@
  *                 \c mbedtls_ssl_recv_t and \c mbedtls_ssl_recv_timeout_t for
  *                 the conventions those callbacks must follow.
  *
- * \note           On some platforms, net.c provides \c mbedtls_net_send(),
- *                 \c mbedtls_net_recv() and \c mbedtls_net_recv_timeout()
- *                 that are suitable to be used here.
+ * \note           On some platforms, net_sockets.c provides
+ *                 \c mbedtls_net_send(), \c mbedtls_net_recv() and
+ *                 \c mbedtls_net_recv_timeout() that are suitable to be used
+ *                 here.
  */
 void mbedtls_ssl_set_bio( mbedtls_ssl_context *ssl,
                           void *p_bio,