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/dns.c b/src/dns.c
index 4fec311..dca2575 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -26,6 +26,7 @@
 #include <haproxy/errors.h>
 #include <haproxy/http_rules.h>
 #include <haproxy/sample.h>
+#include <haproxy/task.h>
 #include <haproxy/time.h>
 #include <haproxy/ticks.h>
 #include <haproxy/net_helper.h>
@@ -43,7 +44,6 @@
 #include <proto/http_ana.h>
 #include <proto/log.h>
 #include <proto/server.h>
-#include <proto/task.h>
 #include <haproxy/proto_udp.h>
 #include <proto/proxy.h>
 #include <proto/stream_interface.h>