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

The type file was slightly tidied. The cli-specific APPCTX_CLI_ST1_* flag
definitions were moved to cli.h. The type file was adjusted to include
buf-t.h and not the huge buf.h. A few call places were fixed because they
did not need this include.
diff --git a/contrib/prometheus-exporter/service-prometheus.c b/contrib/prometheus-exporter/service-prometheus.c
index 50dc14f..f0b1adf 100644
--- a/contrib/prometheus-exporter/service-prometheus.c
+++ b/contrib/prometheus-exporter/service-prometheus.c
@@ -14,6 +14,7 @@
  */
 
 #include <haproxy/action-t.h>
+#include <haproxy/applet.h>
 #include <haproxy/api.h>
 #include <common/cfgparse.h>
 #include <haproxy/frontend.h>
@@ -25,7 +26,6 @@
 #include <haproxy/pool.h>
 #include <haproxy/list.h>
 
-#include <proto/applet.h>
 #include <proto/backend.h>
 #include <haproxy/compression.h>
 #include <haproxy/pipe.h>