REORG: include: move session.h to haproxy/session{,-t}.h

Almost no change was needed beyond a little bit of reordering of the
types file and adjustments to use session-t instead of session at a
few places.
diff --git a/src/stats.c b/src/stats.c
index 1f1e5cd..bc9aaee 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -40,6 +40,7 @@
 #include <haproxy/listener.h>
 #include <haproxy/map-t.h>
 #include <haproxy/pattern-t.h>
+#include <haproxy/session.h>
 #include <haproxy/task.h>
 #include <haproxy/tools.h>
 #include <haproxy/ticks.h>
@@ -61,7 +62,6 @@
 #include <proto/log.h>
 #include <haproxy/pipe.h>
 #include <proto/proxy.h>
-#include <proto/session.h>
 #include <proto/ssl_sock.h>
 #include <proto/stream.h>
 #include <proto/server.h>