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/programs/ssl/dtls_client.c b/programs/ssl/dtls_client.c
index b37eb83..442a3fb 100644
--- a/programs/ssl/dtls_client.c
+++ b/programs/ssl/dtls_client.c
@@ -51,7 +51,7 @@
 
 #include <string.h>
 
-#include "mbedtls/net.h"
+#include "mbedtls/net_sockets.h"
 #include "mbedtls/debug.h"
 #include "mbedtls/ssl.h"
 #include "mbedtls/entropy.h"