Release 0.3.0

The biggest feature from this release is that bwrap
now supports being invoked recursively (from other container
runtimes such as Docker/podman/runc as well as bwrap itself)
when user namespaces are enabled, and the outer container manager
allows it (Docker's default seccomp policy doesn't).

This is useful for testing scenarios; for example a project
uses Kubernetes for its CI, but inside build the project wants to run
each unit test in their own pid namespace, without going out
and creating a new pod for every single unit test.

Similarly, `rpm-ostree compose tree` uses bwrap internally for scripts,
and we want to support running rpm-ostree inside a container as well.

Another feature is bwrap now supports `--`  to terminate argument
parsing.  To detect availablity of this, you could parse `bwrap --version`.

Thanks to all contributors!

```
Colin Walters (3 PRs, 3 commits)
  ci: Update to FAH27 (#262)
  Release 0.3.0 (#277)
  PR: #256
    Use pivot_root() instead of chroot() for final root
    (and 2 commits from other authors)

Giuseppe Scrivano (1 PR, 2 commits)
  PR: #256
    bwrap, pivot_root: do not require write access to the rootfs
    bwrap: do not always make /proc/{sys,sysrq-trigger,irq} ro
    (and 1 commits from other authors)

Olivier Blin (1 PR, 1 commit)
  Fix leak detected by LSan/ASan (#271)

Simon McVittie (1 PR, 1 commit)
  Add "--" pseudo-argument to end option parsing (#261)
```

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