MINOR: checks/vars: Add a check scope for variables

Add a dedicated vars scope for checks. This scope is considered as part of the
session scope for accounting purposes.

The scope can be addressed by a valid session, even embryonic. The stream is not
necessary.

The scope is initialized after the check session is created. All variables are
then pruned before the session is destroyed.
diff --git a/include/types/vars.h b/include/types/vars.h
index 9b166e9..a72469f 100644
--- a/include/types/vars.h
+++ b/include/types/vars.h
@@ -12,6 +12,7 @@
 	SCOPE_REQ,
 	SCOPE_RES,
 	SCOPE_PROC,
+	SCOPE_CHECK,
 };
 
 struct vars {