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/config.h b/include/mbedtls/config.h
index 8d7d631..8a892d7 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1960,7 +1960,7 @@
  * environment:
  * https://tls.mbed.org/kb/how-to/how-do-i-port-mbed-tls-to-a-new-environment-OS
  *
- * Module:  library/net.c
+ * Module:  library/net_sockets.c
  *
  * This module provides networking routines.
  */