[MINOR] config: provide a function to quote args in a more friendly way

The quote_arg() function can be used to quote an argument or indicate
"end of line" if it's null or empty. It should be useful to more precisely
report location of problems in the configuration.
diff --git a/src/standard.c b/src/standard.c
index 3f3e6bd..12954b0 100644
--- a/src/standard.c
+++ b/src/standard.c
@@ -1059,6 +1059,25 @@
 	return addr - cp;
 }
 
+/* To be used to quote config arg positions. Returns the short string at <ptr>
+ * surrounded by simple quotes if <ptr> is valid and non-empty, or "end of line"
+ * if ptr is NULL or empty. The string is locally allocated.
+ */
+const char *quote_arg(const char *ptr)
+{
+	static char val[32];
+	int i;
+
+	if (!ptr || !*ptr)
+		return "end of line";
+	val[0] = '\'';
+	for (i = 1; i < sizeof(val) - 1 && *ptr; i++)
+		val[i] = *ptr++;
+	val[i++] = '\'';
+	val[i] = '\0';
+	return val;
+}
+
 /*
  * Local variables:
  *  c-indent-level: 8