Unconditional use of `getifaddrs`

Issue: [`src/main.rs:214`](https://github.com/sigoden/dufs/blob/b69946df232ceca5d24d0067f2beb6b5ed1c292c/src/main.rs#L214) uses `getifaddrs` unconditionally, even when the input is a fully resolved local address, or a unix domain socket path. This is a bit inconvenient when trying to secure `dufs` with things like systemd and seccomp because `getifaddrs` requires `AF_NETLINK`. Solution if possible: can we move the call to inside the `ip.is_unspecified` branch? I can work on this and make a PR if it sounds good.