crtools: preload libraries (v2)

Libraries (plugins) is going to be used for dumping and restoring
external dependencies (e.g. dbus, systemd journal sockets, charecter
devices, etc)

A plugin can have the cr_plugin_init() and cr_plugin_fini functions for
initialization and deinialization.

criu-plugin.h contains all things, which can be used in plugins.

v2: rename lib to plugin
v3: add a default value for a plugin path.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>