confile: unhide lxc_config_define*() helpers

confile: unhide lxc_config_define*() helpers

Let's unhide lxc_config_define_add, lxc_config_define_load and
lxc_config_define_free helpers. These functions are safe enough
to be used by external tools. Semantic is also clear.

Reason is that we have lxc-start/lxc-execute tools which
use these symbols. Right now it works, because we just
link a whole liblxc statically to each lxc-* tool...

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>