console: clean tty state + return 0 on peer exit

console: clean tty state + return 0 on peer exit

In the past, if the console client exited, lxc_console_cb_con return 1. And
the lxc_poll will exit, the process will wait at waitpid. At this moment, the
process could not handle any command (For example get the container state
LXC_CMD_GET_STATE or stop the container LXC_CMD_STOP.).

I think we should clean the tty_state and return 0 in this case. So, we can use
the lxc-console to connect the console of the container. And we will not exit
the function lxc_polland we can handle the commands by lxc_cmd_process

Reproducer prior to this commit:
- open a new terminal, get the tty device name by command tty /dev/pts/6
- set lxc.console.path = /dev/pts/6
- start the container and the ouptut will print to /dev/pts/6
- close /dev/pts/6
- try an operation e.g. getting state with lxc-ls and lxc-ls will hang

Closes #1787.

Signed-off-by: LiFeng <lifeng68@huawei.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>