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/pkey/dh_client.c b/programs/pkey/dh_client.c
index 8ebf34a..48b97ce 100644
--- a/programs/pkey/dh_client.c
+++ b/programs/pkey/dh_client.c
@@ -37,7 +37,7 @@
     defined(MBEDTLS_ENTROPY_C) && defined(MBEDTLS_NET_C) && \
     defined(MBEDTLS_RSA_C) && defined(MBEDTLS_SHA256_C) && \
     defined(MBEDTLS_FS_IO) && defined(MBEDTLS_CTR_DRBG_C)
-#include "mbedtls/net.h"
+#include "mbedtls/net_sockets.h"
 #include "mbedtls/aes.h"
 #include "mbedtls/dhm.h"
 #include "mbedtls/rsa.h"