Improve kernel argument parsing

[jeckersb]

This adds a new kernel::Cmdline struct, which is populated either
via Cmdline::from (borrowed) or Cmdline::from_proc (owned).

This attempts to follow the same behavior as the kernel, which is
mostly covered in:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/cmdline.c?id=e8d780dcd957d80725ad5dd00bab53b856429bc0#n227

The algorithm is basically:

• Scan forward until you find the first unquoted isspace()
  byte. That's the end of the param.
• If you encounter an = along the way, note where. That's where it
  will terminate the key and split for the value. Any future = are
  not treated as special.
• The value can be quoted to allow spaces, but is unquoted only in as
  much as " is removed from the first or last byte. You can still
  have " in the middle of the value.

This operates on &[u8] because the kernel does not enforce any
particular encoding for the cmdline. Iterating using
Cmdline::iter() will emit the Parameter type, which has helper
methods key_lossy() and value_lossy() to convert
potentially-non-UTF8 data into Strings.

Resolves: #1425
Signed-off-by: John Eckersberg jeckersb@redhat.com

[gemini-code-assist]

Code Review

This pull request introduces a new, more robust kernel command line parser, kernel::Cmdline, which correctly handles quoted values. The new parser is used throughout the codebase, replacing the previous simpler implementation. The changes are well-tested and improve correctness. I've provided a few suggestions in kernel.rs to improve code conciseness and use more idiomatic Rust patterns.

[cgwalters]

Looks sane just some style nits. Thanks for working on this!

[cgwalters]

I think it's a lot cleaner to use split_once for this. There are several other examples in the code.

[jeckersb]

split_once is nightly-only for slices though ☹️

https://doc.rust-lang.org/std/primitive.slice.html#method.split_once

[cgwalters]

Ah yeah, annoying.

You know I think the bigger picture thing here is that almost everyone doing parsing of u8 is using a higher level library and so there's not much motivation to improve std here. There's a lot of those.

One most relevant for this use case is BStr which has https://docs.rs/bstr/latest/bstr/trait.ByteSlice.html#method.split_once_str

bstr is a pretty good fit for this, it's not in our depchain today but that's just because we don't use regex among other things.

OK as is but perhaps worth a comment like

// https://doc.rust-lang.org/std/primitive.slice.html#method.split_once is nightly only right now
// We might consider using bstr in the future too

or so?

[cgwalters]

Not a blocker but something to discuss I think raised by this that we will want to address is that we still have overlap/duplication with at least the kargs parsing in composefs-boot. So one option is to move this there (and also make it pub). The other possibility (which I keep thinking more about) is a "friendly" fork of composefs-boot that lives in this repo; I think we may end up needing that really for things like #1469 among others.