Terminal support is failing on lxc container

on lxc, tty returns "not a tty" although no error is raised. ExecPty considers it a valid tty path and reads/writes on it.
The suggested fix:

• Check that the file exists and is not equals to "not a tty" (just in case...)
• If the file doesn't exist return System.in/out
• Any call to stty -f <file path> fallback already to not use -f so we should rely on it.