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/CMakeLists.txt b/library/CMakeLists.txt
index d990d43..ee5920f 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -89,7 +89,7 @@
     ssl_cookie.c
     ssl_srv.c
     ssl_ticket.c
-    ssl_tls.c
+    ssl_tls_old.c
 )
 
 if(CMAKE_COMPILER_IS_GNUCC)