[Rd] patch for scan.c for long vector support

[Aaron Goodman]

I noticed that scan.c does not yet support long vectors. It was causing a
problem in the rstan library (https://github.com/stan-dev/rstan/issues/530)

I was able to patch base R to add long vector support. See the following
gist.
https://gist.github.com/aaronjg/f39e5966687ca004dab5a10e7655c648

I could not make a bugzilla account, so I am sending the patch here.

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