Add new --level-prefix option

[smcv]

• test-run: Assert that repeating --chdir logs a warning

  This is the case since commit 0d369cd "main: Warn when
  non-repeatable options are repeated".

• utils: Put nearly all diagnostic output through a common log function

  This takes a syslog-style severity level, allowing us to act based on
  the severity.

  Take the opportunity to make the __debug__ macro (which normally expands
  to nothing, but can be enabled by changing a #if 0 to #if 1) less
  weird and easier to use, by taking it out of the reserved-for-the-compiler
  namespace, adding a newline automatically, and not requiring nested
  parentheses.

• Add new --level-prefix option

  This prepends a syslog-style priority level such as <3> to each line of
  diagnostic output, so that the diagnostic output can be parsed by
  tools like systemd-cat --level-prefix=1. A future version of Steam's
  pressure-vessel is likely to use this to make warnings and fatal errors
  from bubblewrap more visible.

cc @refi64 @RyuzakiKK

[smcv]

Updated: while reviewing a similar change in libcapsule, @refi64 pointed out that I'd described the prefixes as "syslog-style" here, but not in libcapsule. In fact saying "syslog-style" is misleading, because https://datatracker.ietf.org/doc/html/rfc5424#section-6.2 describes a prefix that encodes both the severity and the facility, whereas here we're using only the severity, in order to be compatible with systemd-cat(1). Consistently say "severity" in preference to "priority" to reflect this.

(Documentation updates only; the only code change is to rename a level function parameter to severity.)

[smcv]

Any thoughts on this from other maintainers? My colleague @refi64 already provided some useful feedback, which I've addressed.

[RyuzakiKK]

I'm not a maintainer, but FWIW this looks good to me as well.