separate console device from console log

lxc-start -c makes the named file/device the container's console, but using
this with a regular file in order to get a log of the console output does
not work very well if you also want to login on the console. This change
implements an additional option (-L) to simply log the console's output to
a file.

Both options can be used separately or together. For example to get a usable
console and log: lxc-start -n name -c /dev/tty8 -L console.log

The console state is cleaned up more when lxc_delete_console is called, and
some of the clean up paths in lxc_create_console were fixed.

The lxc_priv and lxc_unpriv macros were modified to make use of gcc's local
label feature so they can be expanded more than once in the same function.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>