lxc_setup_ttys: Handle existing ttyN file without underlying device

lxc_setup_ttys: Handle existing ttyN file without underlying device

If a device file is opened and there isn't the underlying device,
the open call fails with ENXIO, but the path can be opened with
O_PATH, which is enough for mounting over the device file.

Generalize this idea and use O_PATH for all cases when the file
is there. One still must check for both ENXIO and EEXIST as it's
unspecified what error is reported if multiple error conditions
occur at the same time.

Signed-off-by: Petr Malat <oss@malat.biz>