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

The TASK_IS_TASKLET() macro was moved to the proto file instead of the
type one. The proto part was a bit reordered to remove a number of ugly
forward declaration of static inline functions. About a tens of C and H
files had their dependency dropped since they were not using anything
from task.h.
diff --git a/include/proto/signal.h b/include/proto/signal.h
index aaa23f6..04dc449 100644
--- a/include/proto/signal.h
+++ b/include/proto/signal.h
@@ -15,7 +15,7 @@
 #include <haproxy/thread.h>
 
 #include <types/signal.h>
-#include <types/task.h>
+#include <haproxy/task-t.h>
 
 extern int signal_queue_len;
 extern struct signal_descriptor signal_state[];