MEDIUM: log: report file name, line number, and directive name with log-format errors

Improve error log reporting in the format parser by always giving the
file name, line number, and directive name instead of the hard-coded
"log-format".

Previously we got this when dealing with log-format errors:
  [WARNING] 101/183012 (8561) : Warning: log-format variable name 'r' is not suited to HTTP mode
  [WARNING] 101/183012 (8561) : log-format: sample fetch <hdr(a,1)> may not be reliably used with 'log-format' because it needs 'HTTP request headers,HTTP response headers' which is not available here.
  [WARNING] 101/183012 (8561) : Warning: no such variable name 'k' in log-format

Now we have this :
  [WARNING] 101/183016 (8593) : parsing [fmt.cfg:8] : 'log-format' variable name 'r' is reserved for HTTP mode
  [WARNING] 101/183016 (8593) : parsing [fmt.cfg:8] : 'log-format' : sample fetch <hdr(a,1)> may not be reliably used here because it needs 'HTTP request headers,HTTP response headers' which is not available here.
  [WARNING] 101/183016 (8593) : parsing [fmt.cfg:15] : no such variable name 'k' in 'unique-id-format'
diff --git a/src/proxy.c b/src/proxy.c
index f909d20..9317774 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -406,7 +406,8 @@
 	}
 	if (curproxy->to_log & (LW_REQ | LW_RESP)) {
 		curproxy->to_log &= ~(LW_REQ | LW_RESP);
-		Warning("config : 'option httplog' not usable with %s '%s' (needs 'mode http'). Falling back to 'option tcplog'.\n",
+		Warning("parsing [%s:%d] : 'option httplog' not usable with %s '%s' (needs 'mode http'). Falling back to 'option tcplog'.\n",
+			curproxy->conf.lfs_file, curproxy->conf.lfs_line,
 			proxy_type_str(curproxy), curproxy->id);
 	}
 	if (curproxy->conf.logformat_string == default_http_log_format ||