add nuse-config.c and nuse.conf instead of env variables for the issue #6

This is the idea 2 of the issue #6.
And, retry of pr #10 .

All existing configurations through env variables are removed and collected to a configuration file.
env NUSECONF specifies a config file for a nuse process.

ex) sudo NUSECONF=nuse.conf ./nuse ping 172.16.0.2

And, syntax of configuration file is shown below.
Multiple interface/route definitions are naturally supported.
```
# interface definition
interface p1p1
        address 172.16.0.1
        netmask 255.255.255.0
        macaddr 00:01:01:01:01:02
        viftype TAP

# route entry definition.
route
        network 0.0.0.0
        netmask 0.0.0.0
        gateway 192.168.0.1
```