reset errno to 0 after calling isatty

isatty(0) also changes errno when returns 0. This causes unexpected error handling.

> RETURN VALUE
>       isatty() returns 1 if fd is an open file descriptor referring to a terminal; otherwise 0 is returned, and errno is set to indicate the error.

I encounterd an error caused by this issue at  https://github.com/armon/statsite/blob/f7935745181ade7a36302f4fa5f8a63ce18a5fea/src/config.c#L82

I set *port*  to 0. Althouch strtol did not throw an error, errno had been set to EINVAL by isatty, and value_to_int returns 0 consequently.