Work around a new nightly clippy warning

The warning clippy::bool_to_int_with_if is meant to shout at you when you say if x { 1 } else { 0 } and instead suggest that you say inttype::from(x).

I agreed with this for the case in tor-cert, where we are literally converting a boolean into a flag.

I don't agree with this in tor-netdoc, where we are using a boolean to decide how many fields to skip in a given document format. So for this case, I decided to clean up the code a little by renaming "skip" to "n_skip", and changing the boolean to use an enum instead.