compel cli: add plugins cmd, use it

A compel user should not be aware of compel internals, including
the paths to various files. This commit introduces a command to
get the plugin files (well, currently just one plugin, "std").

Example for uninstalled compel:
 $ cd compel/test/infect/
 $ ../../../compel/compel-host plugins
 ../../../compel/plugins/std.built-in

Example for installed compel:
 $ compel plugins
 /usr/libexec/compel/std.built-in.o

The commit also makes use of this command by compel/test/infect
and criu/pie.

travis-ci: success for More polishing for compel cli
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>