lxc_create: support 'lxc-create -t <template> -h'

With the lxc-create script, 'lxc-create -t template -h' used to call
'template -h' to get template-specific help.  The api based lxc-create
did not yet support that.

Add a 'helpfn' method to the lxc_arguments, which is called at the end
of printhelp, and passed the lxc_arguments.  Use that in lxc_create to
reintroduce the desired behavior.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>