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/src/cfgparse.c b/src/cfgparse.c
index a0178e6..6b27c24 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -6518,10 +6518,12 @@
 		}
 
 		if (curproxy->logformat_string)
-			parse_logformat_string(curproxy->logformat_string, curproxy, &curproxy->logformat, LOG_OPT_MANDATORY);
+			parse_logformat_string(curproxy->logformat_string, curproxy, &curproxy->logformat, LOG_OPT_MANDATORY,
+					       SMP_VAL_FE_LOG_END);
 
 		if (curproxy->uniqueid_format_string)
-			parse_logformat_string(curproxy->uniqueid_format_string, curproxy, &curproxy->format_unique_id, 0);
+			parse_logformat_string(curproxy->uniqueid_format_string, curproxy, &curproxy->format_unique_id, 0,
+					       (proxy->cap & PR_CAP_FE) ? SMP_VAL_FE_HRQ_HDR : SMP_VAL_BE_HRQ_HDR);
 
 		/* first, we will invert the servers list order */
 		newsrv = NULL;