MAJOR: acl: make all ACLs reference the fetch function via a sample.

ACL fetch functions used to directly reference a fetch function. Now
that all ACL fetches have their sample fetches equivalent, we can make
ACLs reference a sample fetch keyword instead.

In order to simplify the code, a sample keyword name may be NULL if it
is the same as the ACL's, which is the most common case.

A minor change appeared, http_auth always expects one argument though
the ACL allowed it to be missing and reported as such afterwards, so
fix the ACL to match this. This is not really a bug.
diff --git a/include/proto/sample.h b/include/proto/sample.h
index 3137948..036ac76 100644
--- a/include/proto/sample.h
+++ b/include/proto/sample.h
@@ -35,5 +35,6 @@
 void sample_register_fetches(struct sample_fetch_kw_list *psl);
 void sample_register_convs(struct sample_conv_kw_list *psl);
 const char *sample_src_names(unsigned int use);
+struct sample_fetch *find_sample_fetch(const char *kw, int len);
 
 #endif /* _PROTO_SAMPLE_H */