lxc_monitor_open: prepend lxcpath

This is needed for lxc_wait and lxc_monitor to handle lxcpath.  However,
the full path name is limited to 108 bytes.  Should we use a md5sum of
the lxcpath instead of the path itself?

In any case, with this patch, lxc-wait and lxc-monitor work right with
respect to multiple lxcpaths.

The lxcpath is added to the lxc_handler to make it available most of the
places we need it.

I also remove function prototypes in monitor.h for two functions which
are not defined or used anywhere.

TODO: make cgroups tolerate multiple same-named containers.

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