[EVAL] Let funcnode refer to a function definition, not its first command

It is not unrelated: I changed the meaning of struct funcnode's field n
to refer to the function definition, rather than the list of the
function's commands, because I needed to refer to the function
definition node from evalfun, which only gets passed a funcnode. But it
is something that could be applied independently (without being useful
by itself), so I've attached it as a separate patch for easier review.

Signed-off-by: Harald van Dijk <harald@gigawatt.nl>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>