chore: canonical exe path before validating.

[jmcarp]

We validate that the dpd path lives at the expected location on start. However, this check can throw a false positive if we point to the exe via a symlink. This patch canonicalizes the exe path before validating so that we validate the real exe path, which may differ from env::current_exe().

Submitted to avoid a PATH workaround for running simulated omicron on macos. h/t @david-crespo.

[jmcarp]

For context, I'm trying to run simulated omicron on my macbook, and getting a validation error from the dpd stub:

bash-3.2$ dpd run

thread 'main' panicked at dpd/src/main.rs:264:23:
unable to initialize bf: P4Missing("/Users/joshcarp/code/omicron/out/dendrite-stub expected to end with dendrite")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Abort trap: 6

Because the dpd stub on my PATH is a symlink:

bash-3.2$ which dpd
./out/dendrite-stub/bin/dpd
bash-3.2$ realpath $(which dpd)
/Users/joshcarp/code/omicron/out/dendrite-stub/root/opt/oxide/dendrite/bin/dpd

If I change my PATH to point to the symlinked directory directly, it works, but it occurred to me that this would be simpler if dpd checked its canonical path for validation instead.

@david-crespo pointed out that this seems to be a macos-specific issue, since we aren't aware of the same problem on linux. I think canonicalizing the exe path is probably reasonable either way, but I'm wondering why this hasn't come up on linux so far.