Release 0.1.8

This release has a new notable feature in `--die-with-parent`,
which is based on the Linux `prctl(PR_SET_PDEATHSIG)` API.
I suspect most users of bwrap probably want to use this - if
for example if you run `bwrap ... make check`, this will help
ensure that no processes leak from the test suite.

Besides that, there's mostly a collection of smaller bugfixes.

Thanks to all contributors!

```
Aidan Hobson Sayers (2):
      Remove privileged_op flags that are never used
      Correctly validate remount-ro argument

Aleksa Sarai (1):
      README: update references to runC

Colin Walters (8):
      build: Remove unbalanced ) in help message
      tests: Use --unshare-user-try
      ci: Revamp to actually run the tests
      Be more informative if loopback setup fails
      tests: Fold test-basic.sh into test-run.sh
      ci: Disable ASAN leak checking
      main: Parse --version early before acquiring capabilities
      Release 0.1.8

Giuseppe Scrivano (1):
      test-run.sh: fix the path for the usage string

Marek Jarycki (1):
      Add --die-with-parent

Mario Sanchez Prada (1):
      Ignore EPERM when dropping caps from bounding set

Tristan Cacqueray (1):
      Ignore missing sysrq-trigger file

valoq (2):
      Add --require-userns build option for setuid mode
      Added --unshare-all to manpage
```

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