hakurei 0.3.0

Migration:
The `cmd/hsu` uid format has changed and is now stabilised in package `hst`.
Existing installations need to migrate by manually running `chown` recursively
over existing application data directories. The uid format can be found in
[hst/grp_pwd.go](https://git.gensokyo.uk/security/hakurei/src/tag/v0.3.0/hst/grp_pwd.go).

Additionally, `TMPDIR` and `XDG_RUNTIME_DIR` sharing between containers under
the same identity is now optional and defaults to *off*. This is *not* an
implementation detail. Some applications use `TMPDIR` for communication and
state tracking. Chromium is a good example of this. Test your apps, if they
malfunction, set `share_runtime` or `share_tmpdir` to true depending on the
cause.

API Changes:
- Remove raw seccomp flags from package `hst`.
- All boolean fields in `ContainerConfig` are packed into a single integer.
  Does not break json.
- Package `system` now operates exclusively on absolute pathnames.
- `ProxyDBus` no longer returns function to dump buffer.
- `Wayland` no longer returns the security context close file.
- Container related fields are moved from the top level `Config` struct to
  `ContainerConfig`.
- Pathnames, bits, seccomp types are relocated from package `container`.
- Cross-platform `json` adaptor provided for `seccomp` types.
- Message and check types are relocated from package `container`.
- The `wait_delay` field now takes a negative value. Such a value is equivalent
  to zero and bypasses default.
- Rename `hst.AbsTmp` to `hst.AbsPrivateTmp`.
- Remove `PlaceP` method from package `container`.
- Remove `system/tmpfiles` as it is no longer used.
- Hold `hst.ExtraPermConfig` by value instead of address.
- Instance state now embeds `hst.Config`.
- State store entries are prefixed with a versioned header encoding the
  enablements byte.
- `container/params` now exposes the write end of setup pipe.
- `hakurei --json ps` serialises to an array instead of a map.
- Rename `NewMsg` to `New` in package `message`.

Fixes:
- Do not incorrectly offset shim timeout base value.
- Apply configured filesystems late, enabling system mount points to be covered.
- Check `nscd` socket for path hiding instead of only for pd.
- Use correct well-known address for `dbus` fallback.
- Fix synchronisation order in `container` and `helper` tests.
- Do not leak state (outside of dispatcher) in `system` builder methods.
- Ignore nondeterministic mount point in vm test.
- Fix occasional incorrect reporting of `wstatus` in `container/init`.
- Use `EOF` as failsafe in `internal/outcome/shim`.
- Fallback runtime dir is no longer rooted in the fallback rundir.
- State is now tracked in `TMPDIR`, aligning better with use case detailed in
  [the XDG basedir spec](https://specifications.freedesktop.org/basedir/0.8/#variables).
- Make application identifier string `omitempty`.
- Handle nil `hst.Config` address in `cmd/hakurei/print`.

Enhancements:
- Package `hst` now builds without cgo.
- Significantly improve `hst` doc comments.
- Stable enablements bits, instance state, exit codes, uid format in `hst`.
- Validate `hst.Config` early and export validation function.
- Optionally share `TMPDIR` and `XDG_RUNTIME_DIR` between containers under the
  same identity.
- Dump `system/dbus` buffer internally.
- Hang up `system/wayland` security context internally.
- Only minimal pathnames required for `hst.Paths` are sent.
- Outcome finalisation is fully modularised and tested.
- Container is now finalised on the shim side, significantly reducing transfer
  size.
- Outcome finalisation no longer clobbers supplied `hst.Config`.
- Use more plausible `overlay` mount configuration in `hst.Template`.
- Remove confusing or redundant/noisy entries in `hst.Template`.
- Eliminate the self pipe and copy `seccomp` program into managed memory.
- Emulate `pam_systemd` `XDG_SESSION_TYPE` behaviour according to enablement
  bits.
- Hakurei version is now exposed via `hst`.
- Print cleaner `json` error messages from `cmd/hakurei`.
- Improve `cmd/hakurei` container flags string representation.
- Export `prctl` wrapper.
- Improve signal handling in `container/init`.
- Do not suspend output in `container/init` and `internal/outcome/shim`.
- Store priv-side pid in state store entries.
- Measure duration of finalise and each process state.

Internal:
- Almost all tests are parallelised.
- Remove `internal/hlog`.
- Remove global `Msg` implementation from `container`.
- Package `internal/app` is renamed to `internal/outcome` and cleaned up, with
  some of its code relocated to independent packages.
- Permissive defaults behaviour is eliminated and relocated to `cmd/hakurei`
  run command.
- Clean up legacy `internal/outcome` integration tests.
- Check against setgid bit in `cmd/hsu`.
- Use much less ambiguous names which are set by the linker.
- The `internal/store` interface is significantly improved, with much better
  efficiency and lower lock contention.
- The `lockedfile` package is relocated from the tool chain, and can be used by
  project internals going forward.
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