MEDIUM: cache: Change caching conditions

Do not cache responses that do not have an explicit expiration time
(s-maxage or max-age Cache-Control directives or Expires header) or a
validator (ETag or Last-Modified headers) anymore, as suggested in
RFC 7234#3.
The TX_FLAG_IGNORE flag is used instead of the TX_FLAG_CACHEABLE so as
not to change the behavior of the checkcache option.
diff --git a/reg-tests/cache/basic.vtc b/reg-tests/cache/basic.vtc
index e8255af..849057d 100644
--- a/reg-tests/cache/basic.vtc
+++ b/reg-tests/cache/basic.vtc
@@ -6,7 +6,8 @@
 
 server s1 {
     rxreq
-    txresp -nolen -hdr "Transfer-Encoding: chunked"
+    txresp -nolen -hdr "Transfer-Encoding: chunked" \
+        -hdr "Cache-Control: max-age=5"
     chunkedlen 1
     chunkedlen 1
     chunkedlen 2