init: log resolved datadir path when it differs #36373

If the data directory (or a parent) is a symlink, also log the path it resolves to, so debug.log files show the actual location for that run.

This patch would mostly help developers. I've been testing a potential performance PR across different data directories (for example, SSD versus HDD). Yes, we can specify -datadir= on the command line, but if a script starts bitcoind (e.g., on an RPi with a whole infrastructure, like Umbrel or myNode), changing the startup command arguments may be inconvenient or not even possible. But it's easy to change the default directory to be a symlink.

If the resolved directory matches the specified directory, there's no need to log both (so no change from today). On macOS, /tmp and /var are symlinks into /private, so a datadir under those (like the functional test temp dirs) would show both.

I don't think a separate test is needed. I haven't tested on all platforms, but this patch uses primitives already used elsewhere, so it should work everywhere.

Here's a before and after example; this line occurs within the first few lines after startup (omitting the timestamp prefix):

- Using data directory /home/larry/.bitcoin
+ Using data directory /home/larry/.bitcoin (/sd/.bitcoin)

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36373
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36373.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36373#author-ralph

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A better approach might be to print a separate line, instead of changing the existing one, in case there are scripts out there that grep for that line and extract the directory path by cutting everything up to a particular column (or taking everything after the third token). So the resolved path could immediately follow the line Using path:

Resolved data directory /sd/.bitcoin

I'd recommend always printing that line (not only if it differs from the nominal path) to simplify scripts that want to grab the resolved path (they don't have to search for "Resolved..." and, if not found, revert to "Using..."). Not much harm in printing one extra line.