use a default per-container logfile

[ Thanks to Stéphane and Dwight for the feedback on the previous patch ]

Until now, if a lxc-* (i.e. lxc-start) command did not specify a logfile
(with -o logfile), the default was effectively 'none'.  With this patch,
the default becomes $LOGPATH/<container>/<container>.log.  LOGPATH is
specified at configure time with '--with-log-path='.  If unspecified, it
is $LXCPATH, so that logs for container r2 will show up at
/var/lib/lxc/r2/r2/log.  LOGPATH must exist, while lxc will make sure to
create $LOGPATH/<name>.  As another example, Ubuntu will likely specify
--with-log-path=/var/log/lxc (and place /var/log/lxc into
debian/lxc.dirs), placing r2's logs in /var/log/lxc/r2/r2.log.

If a container config file specifies 'lxc.logfile', that will override
the default.  If a '-o logfile' argument is specifed at lxc-start,
then that will override both the default and the configuration file
entry.  Finally, '-o none' can be used to avoid having a logfile at
all (in other words, the previous default), and that will override
a lxc.logfile entry in the container configuration file.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
use a default per-container logfile

log_open: make sure the parent directory for logfiles

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>