use a default per-container logfile (v2)

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 a per-container log file.

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.

If the user does not have rights to open the default, then 'none' will
be used.  However, in that case an error will show up on console.  (We
can work on removing that if it annoys people, but I think it is
helpful, at least while we're still ironing this set out)  If the user
or container configuration file specified a logfile, and the user does
not have rights to open the default, then the action will fail.

changelog:  Jan 24:

 add --enable-configpath-log configure option

 When we log to /var/lib/lxc/$container/$container.log, several things
 need to be done differently than when we log into /var/log/lxc (for
 instance).  So give it a configure option so we know what to do

 When the user specifies a logfile, we bail if we can't open it.  But
 when opening the default logfile, the user may not have rights to
 open it, so in that case ignore it and continue as if using 'none'.
 (For instance, unprivileged user using lxc-info)

 When using /var/lib/lxc/$c/$c.log, we use $LOGPATH/$name/$name.log.
 Otherwise, we use $LOGPATH/$name.log.

 When using /var/lib/lxc/$c/$c.log, don't try to create the log path
 /var/lib/lxc/$c.  It can only not exist if the container doesn't
 exist.  We don't want to create the directory in that case.  When
 using /var/log/lxc, then we do want to create the path if it does
 not exist.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>