MINOR: errors: specify prefix "config" for parsing output

Set "config :" as a prefix for the user messages context before starting
the configuration parsing. All following stderr output will be prefixed
by it.

As a consequence, remove extraneous prefix "config" already specified in
various ha_alert/warning/notice calls.
diff --git a/src/mux_h1.c b/src/mux_h1.c
index dd65460..c06cec9 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -3696,7 +3696,7 @@
 
 	file = fopen(hdrs_map.name, "r");
 	if (!file) {
-		ha_alert("config : h1-outgoing-headers-case-adjust-file '%s': failed to open file.\n",
+		ha_alert("h1-outgoing-headers-case-adjust-file '%s': failed to open file.\n",
 			 hdrs_map.name);
                 err_code |= ERR_ALERT | ERR_FATAL;
 		goto end;
@@ -3745,14 +3745,14 @@
 		err = NULL;
 		rc = add_hdr_case_adjust(key_beg, value_beg, &err);
 		if (rc < 0) {
-			ha_alert("config : h1-outgoing-headers-case-adjust-file '%s' : %s at line %d.\n",
+			ha_alert("h1-outgoing-headers-case-adjust-file '%s' : %s at line %d.\n",
 				 hdrs_map.name, err, line);
 			err_code |= ERR_ALERT | ERR_FATAL;
 			free(err);
 			goto end;
 		}
 		if (rc > 0) {
-			ha_warning("config : h1-outgoing-headers-case-adjust-file '%s' : %s at line %d.\n",
+			ha_warning("h1-outgoing-headers-case-adjust-file '%s' : %s at line %d.\n",
 				   hdrs_map.name, err, line);
 			err_code |= ERR_WARN;
 			free(err);