clean exit on EPOLLHUP

clean exit on EPOLLHUP

lxc_console_cb_tty_masterfd() unnecessarily reported a read/write error when
the fd was closed. This happens e.g. when we have allocated a tty in the
container with lxc-console and we shut the container down. lxc-console will
then exit with an error message. This patch introduces a test whether the
EPOLLHUP bit is set in the events mask. If so, we report no error.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>