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/src/frontend.c b/src/frontend.c
index 44ac151..4066fe9 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -26,6 +26,7 @@
 #include <haproxy/chunk.h>
 #include <haproxy/frontend.h>
 #include <haproxy/sample.h>
+#include <haproxy/task.h>
 #include <haproxy/tools.h>
 #include <haproxy/time.h>
 
@@ -41,7 +42,6 @@
 #include <proto/proxy.h>
 #include <proto/stream.h>
 #include <proto/stream_interface.h>
-#include <proto/task.h>
 
 /* Finish a stream accept() for a proxy (TCP or HTTP). It returns a negative
  * value in case of a critical failure which must cause the listener to be