MEDIUM: action: add a new flag ACT_FLAG_FIRST

This flag is used by custom actions to know that they're called for the
first time. The only case where it's not set is when they're resuming
from a yield. It will be needed to let them know when they have to
allocate some resources.
diff --git a/include/types/action.h b/include/types/action.h
index b1e19e7..ca0163f 100644
--- a/include/types/action.h
+++ b/include/types/action.h
@@ -50,6 +50,7 @@
 enum act_flag {
 	ACT_FLAG_NONE  = 0x00000000,  /* no flag */
 	ACT_FLAG_FINAL = 0x00000001,  /* last call, cannot yield */
+	ACT_FLAG_FIRST = 0x00000002,  /* first call for this action */
 };
 
 enum act_name {