daemon/config: factor out duplicate xstrdup_tolower

We have two implementations of the same function; let's drop
that to one. We take the name from daemon.c, but the
implementation (which is just slightly more efficient) from
the config code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -570,3 +570,16 @@ int fprintf_ln(FILE *fp, const char *fmt, ...)
 		return -1;
 	return ret + 1;
 }
+
+char *xstrdup_tolower(const char *string)
+{
+	char *result;
+	size_t len, i;
+
+	len = strlen(string);
+	result = xmalloc(len + 1);
+	for (i = 0; i < len; i++)
+		result[i] = tolower(string[i]);
+	result[i] = '\0';
+	return result;
+}