Better project options

The current project option setup is simple. All options must be specified at the top level `meson_options.txt` file. We should instead be able to provide roughly similar level of options as kconfig. These include:
- some options that are not shown, for example if you have `win`, `linux` and `osx` subdirectories, each with options, only options from one of them should be added to the set of all options and shown to the user
- arranging options in a tree
- something like `make allmodconfig` seems useful but hard to do reliably with all of the above

KConfig is nice but has severe drawbacks such as being able to set configurations that are broken or inconsistent. We do not want that.

Start by reading the existing project option setup around the top-level meson_options.txt file and compare its behavior with the KConfig capabilities described here. Define how conditional, hidden, hierarchical, and automatically generated options should work while preserving valid configurations; the issue does not name specific implementation files or tests.

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