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/backend.c b/src/backend.c
index 18ac583..02f8e86 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -31,6 +31,7 @@
 #include <haproxy/lb_fwrr.h>
 #include <haproxy/obj_type.h>
 #include <haproxy/payload.h>
+#include <haproxy/task.h>
 #include <haproxy/ticks.h>
 #include <haproxy/time.h>
 #include <haproxy/namespace.h>
@@ -55,7 +56,6 @@
 #include <proto/stream.h>
 #include <proto/stream_interface.h>
 #include <proto/ssl_sock.h>
-#include <proto/task.h>
 
 #define TRACE_SOURCE &trace_strm