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/Makefile b/library/Makefile
index 00528b3..4b29628 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -68,9 +68,10 @@
 		x509_create.o	x509_crl.o	x509_crt.o	\
 		x509_csr.o	x509write_crt.o	x509write_csr.o
 
-OBJS_TLS=	debug.o		net.o		ssl_cache.o	\
-		ssl_ciphersuites.o		ssl_cli.o	\
-		ssl_cookie.o	ssl_srv.o	ssl_ticket.o	\
+OBJS_TLS=	debug.o		net_sockets.o		\
+		ssl_cache.o	ssl_ciphersuites.o	\
+		ssl_cli.o	ssl_cookie.o		\
+		ssl_srv.o	ssl_ticket.o		\
 		ssl_tls.o
 
 .SILENT: