add console to lxc api

Make lxc_cmd_console() return the fd from the socket connection to the
caller. This fd keeps the tty slot allocated until the caller closes
it. Returning the fd allows for a long lived process to close the fd
and reuse consoles.

Add API function for console allocation.

Create test program for console API.

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