This adds lxc_config_item_is_supported() as API extension. It allows to check
whether a given config item (e.g. lxc.autodev) is supported by this LXC
instance. The function is useful in the following scenarios:
1. Users have compiled liblxc from source and have removed a config items from
   the corresponding struct in confile.c. (For example, embedded users might
   decide to gut a bunch of options that they cannot use.)
2. Callers that want to check for a specific configuration item independent of
   the version numbers exposed in our version.h header.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>