MINOR: log: indicate it when some unreliable sample fetches are logged

If a log-format involves some sample fetches that may not be present at
the logging instant, we can now report a warning.

Note that this is done both for log-format and for add-header and carefully
respects the original fetch keyword's capabilities.
diff --git a/include/proto/log.h b/include/proto/log.h
index 08a9a1b..86b4185 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -70,7 +70,7 @@
  * Variable name are preceded by % and composed by characters [a-zA-Z0-9]* : %varname
  * You can set arguments using { } : %{many arguments}varname
  */
-void parse_logformat_string(const char *str, struct proxy *curproxy, struct list *list_format, int options);
+void parse_logformat_string(const char *str, struct proxy *curproxy, struct list *list_format, int options, int cap);
 /*
  * Displays the message on stderr with the date and pid. Overrides the quiet
  * mode during startup.