lxc-info: Rework based on mailinglist thread

So this implements the changes we discussed yesterday:
 - Only one container may be queried at the time
 - -n is now required once again
 - -H + a single filter only returns the value
 - -t/--is-state is now removed

Note that -S is considered as more than a single filter, so -H in that
case only affects the formatting of the values.

For the same reason, I haven't yet implemented the -H + multiple filters
case which we said should return a simple "key: value" output as it
wasn't trivial to re-arrange the stats code to print a different format
(for the other options, it's just a two lines change in the print
functions).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Dwight Engen <dwight.engen@oracle.com>