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_server.c b/programs/ssl/dtls_server.c
index 1d6eb3b..9d0dda4 100644
--- a/programs/ssl/dtls_server.c
+++ b/programs/ssl/dtls_server.c
@@ -67,7 +67,7 @@
 #include "mbedtls/x509.h"
 #include "mbedtls/ssl.h"
 #include "mbedtls/ssl_cookie.h"
-#include "mbedtls/net.h"
+#include "mbedtls/net_sockets.h"
 #include "mbedtls/error.h"
 #include "mbedtls/debug.h"
 #include "mbedtls/timing.h"