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

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.