linkchecker: running from a directory separate from the book

Since rust-lang/cargo#11851, Cargo became a Cargo workspace of
itself. However, since `src/tools/linkchecker` cannot run inside
a workspace, Cargo needs a workaround that excludes `src/doc`
from workspace member probing.

To remove this hack, this PR adds a new optional argument `--path`
for `linkchecker.sh`. With this new argument, `linkchecker.sh` can
be run from a directory separate from the book. This also benefits
other projects using linkchecker, as they can run it under target
directory or any other directory, reducing leftover.

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