Add a non-interactive mode to tinc commands.

Some tinc commands, such as "tinc generate-keys", use the terminal to
ask the user for information. This can be bypassed by making sure
there is no terminal, which is trivial on *nix but might require
jumping through some hoops on Windows depending on how the command is
invoked.

This commit adds a --batch option that ensures tinc will never ask the
user for input, even if it is attached to a terminal.