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/channel.h b/include/proto/channel.h
index 3547581..96bf212 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -40,7 +40,7 @@
 #include <types/stream_interface.h>
 
 #include <proto/stream.h>
-#include <proto/task.h>
+#include <haproxy/task.h>
 
 /* perform minimal intializations, report 0 in case of error, 1 if OK. */
 int init_channel();