Display a warning when dune builds via RPC

[gridbugs]

When an instance of dune is already running, invoking dune build will still build the target, howwever other command line arguments are silently ignored by dune as the current RPC interface is too limited to express them. This may change over time, but currently it's unusual UX for dune build to behave differently depending on whether or not another instance of dune build is already running. To more accurately set user expectations, print a warning when dune is running with this reduced functionality.

[nojb]

A couple of questions:

• "If this is not the case, please delete ..." is this relevant ? if the build is being forwarded to a running instance, it doesn't make much sense to me to ask the user to delete the lock file.
• "functionality will be reduced" what does this refer to?

I suggest tweaking the message and making it a bit less scary:

Warning: your build request is being forwarded to a running Dune instance (pid: 124305). Note that certain command line arguments may be ignored.

[Leonidas-from-XIV]

I like the idea of warning the user when flags get ignored, but I would suggest that this would only happen if there are flags that will get discarded.

[gridbugs]

Updated so that it only prints the warning if extra arguments were passed to dune build. This is done by comparing the Builder.t parsed from command-line arguments to the default Builder.t that would be created if no arguments were passed. This has become a fairly big change because I needed to propagate equal functions throughout the code, and I needed to change how Log.File.t is defined so that it can be tested for equality.

I also added Term.eval_no_args for extracting the default value of a type by running its command-line argument parser with no input which is possibly a controversial way of getting a default value, but I think otherwise computing a default value of a Builder.t would require quite a big refactor.

[Leonidas-from-XIV]

It's a bit sad all these very verbose equal functions need to be added but I understand the idea.

Overall I think it needs a small wording change and an additional test and then it is good to go.

[Leonidas-from-XIV]

Since this warning is now only emitted when arguments have been ignored, I think it should state that command line arguments have in fact been ignored.

[gridbugs]

Not all arguments are ignored though. The Builder module is perhaps poorly named, as Builder.term takes care of parsing arguments like --verbose and --display which still take effect when dune is behaving as an rpc client. I'll still think about how to reword this message to be more helpful though.

[Leonidas-from-XIV]

But if I understand it correctly at this point the Builder instance is unequal to one without arguments, thus at this point we know for a fact that arguments have been ignored? Or do I misunderstand the logic of this PR?