Duplicate ssl_tls.c as ssl_tls_old.c and ssl_msg.c

This commit is the first in a series of commits aiming to split
the content of ssl_tls.c in two files ssl_tls.c and ssl_msg.c.

As a first step, this commit replaces ssl_tls.c by two identical
copies ssl_tls_old.c and ssl_msg.c. Even though the file
ssl_tls_old.c will subsequently be renamed back into ssl_tls.c,
this approach retains the git history in both files.
diff --git a/library/Makefile b/library/Makefile
index 6fc110c..072002b 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -78,7 +78,7 @@
 		ssl_cache.o	ssl_ciphersuites.o	\
 		ssl_cli.o	ssl_cookie.o		\
 		ssl_srv.o	ssl_ticket.o		\
-		ssl_tls.o
+		ssl_tls_old.o
 
 INCLUDING_FROM_MBEDTLS:=1
 include ../crypto/3rdparty/Makefile.inc