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/library/CMakeLists.txt b/library/CMakeLists.txt
index 6aeb385..98fe8c9 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -71,7 +71,7 @@
 
 set(src_tls
     debug.c
-    net.c
+    net_sockets.c
     ssl_cache.c
     ssl_ciphersuites.c
     ssl_cli.c