MEDIUM: acl: have a pointer to the keyword name in acl_expr

The acl_expr struct used to hold a pointer to the ACL keyword. But since
we now have all the relevant pointers, we don't need that anymore, we just
need the pointer to the keyword as a string in order to return warnings
and error messages.

So let's change this in order to remove the dependency on the acl_keyword
struct from acl_expr.

During this change, acl_cond_kw_conflicts() used to return a pointer to an
ACL keyword but had to be changed to return a const char* for the same reason.
diff --git a/include/proto/acl.h b/include/proto/acl.h
index 39cbf46..edd4648 100644
--- a/include/proto/acl.h
+++ b/include/proto/acl.h
@@ -110,7 +110,7 @@
  * null), or false if not conflict is found. The first useless keyword is
  * returned.
  */
-int acl_cond_kw_conflicts(const struct acl_cond *cond, unsigned int where, struct acl const **acl, struct acl_keyword const **kw);
+int acl_cond_kw_conflicts(const struct acl_cond *cond, unsigned int where, struct acl const **acl, char const **kw);
 
 /*
  * Find targets for userlist and groups in acl. Function returns the number