lxc-attach: error on -L/--pty-log + redirection
lxc-attach: error on -L/--pty-log + redirection

pty logging only works correctly when stdout and stderr refer to a pty. If they
do not, we do not dup2() them and lxc_console_cb_con() will never write to the
corresponding log file descriptor.

When redirection on stdout and stderr is used we can safely assume that the user
is already logging to a file or /dev/null and creating an additional pty log
doesn't seem to make sense.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@mailbox.org>

ss1h2a3tw/lxc,QingweiPeterLan/cs188-lxc,LynxChaus/lxc,hfuCN/lxc,GreatFruitOmsk/lxc,ysbnim/lxc,aeris/lxc,terceiro/lxc,cjwatson/lxc,ysbnim/lxc,aeris/lxc,thmo/lxc,hallyn/lxc,aeris/lxc,QingweiPeterLan/cs188-lxc,akshaykarle/lxc,ss1h2a3tw/lxc,cjwatson/lxc,caglar10ur/lxc,terceiro/lxc,GreatFruitOmsk/lxc,QingweiPeterLan/cs188-lxc,terceiro/lxc,thmo/lxc,hallyn/lxc,caglar10ur/lxc,cjwatson/lxc,hallyn/lxc,caglar10ur/lxc,hfuCN/lxc,akshaykarle/lxc,ysbnim/lxc,LynxChaus/lxc,GreatFruitOmsk/lxc,federicobriata/lxc,thmo/lxc,hfuCN/lxc,federicobriata/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,federicobriata/lxc,akshaykarle/lxc