Restore tty I/O attributes when daemonizing

[dancek]

Fixes #909, ie. when sudo has the option use_pty enabled, running sshuttle --daemon would previously leave the terminal in a weird state.

[dancek]

I don't fully understand why the daemonize function overwrites stdin and stdout with a null descriptor. And overall my understanding of the process hierarchy in sshuttle is limited. So this may not be the perfect place to restore terminal attributes, but it does work in my experience.

To replicate the issue, run with --daemon and check stty output afterwards. Sudo needs to have use_pty enabled for the issue to happen, ie. Defaults use_pty in /etc/sudoers or a recent version of sudo.

Before running the program:

$ stty
speed 38400 baud; line = 0;

Without this commit:

$ ./run --daemon [my-opts]
$ stty
speed 38400 baud; line = 0;
                           min = 1; time = 0;
                                             -brkint -icrnl
                                                           -opost
                                                                 -isig -icanon -iexten -echo

With this commit:

$ ./run --daemon [my-opts]
$ stty
speed 38400 baud; line = 0;

[skuhl]

I took your code here and did some hunting to pinpoint when the terminal settings were getting garbled. It seems to be related to the socketpair that the firewall and client process use to communicate. I think I found an alternative fix that changes very few lines of code, avoids the garbled terminal in the first place, and fixes something that bothered me in the original fix for the use_pty change: That we had to use \r\n instead of \n at the end of messages to prevent verbose messages from being garbled (even when not in daemon mode). I'll try to post it a bit later today.

[dancek]

This is obsoleted by #923, which fixes the same issue by removing the root cause.