Consolidate TCP-specific common code into their own files.

The TCP keepalive tuning code has been moved into the newly added
files; this also allows future TCP-specific code to be added into
these files, without bloating the IP level code and establishes a
known file structure for other IP-based transports.

Remember: this is a no-op change, hence no API or functionality
was changed as part of this commit.

src/tcp_connecter.cpp

@@ -29,6 +29,7 @@
 #include "random.hpp"
 #include "err.hpp"
 #include "ip.hpp"
+#include "tcp.hpp"
 #include "address.hpp"
 #include "tcp_address.hpp"
 #include "session_base.hpp"