When we merged TOR_RUST_VERSION from master, the allow_failures rule
did not match any more. Update it to make it match.

Closes 31859 for master.

Merge master's RUST_VERSION="nightly" into the macOS Rust job
from bug31859_041.

Drop master's RUST_VERSION="beta", because we only have two
Rust jobs now.

Merge TOR_TEST_RNG_SEED in maint-0.4.1 into the coverage line
from bug31859_040.

Since Rust on macOS is slow, don't wait for the macOS Rust job to finish.
Instead, split rust into slow rust (macOS) and fast rust (Linux). And
allow the build to finish before slow rust finishes.

Also make sure that we have:
* a Rust build on each platform,
* a Rust build with each compiler, and
* a check on all our Rust builds.

Finally, sort builds: allow fail last, macOS first, slowest first.

Closes 31859 for 0.3.5.

Part of 31859 for 0.3.5.

While merging:
* leave out some redundant jobs and build matrix entries

Since Travis macOS has IPv6 support (and Travis Linux does not), chutney
will now run its IPv6 networks as part of Travis CI.

But since chutney is slow, don't wait for the macOS chutney to finish.
(Travis have fixed the duplicate notification bug in fast_finish. So we
can use fast_finish and allow_failure to finish early. Unfortunately,
allow_failure also means we ignore failures in macOS chutney.)

Also make sure that we have:
* a compile on each platform, with each compiler,
* a check on each platform, and
* a check on each compiler.

Finally, sort builds: allow fail last, macOS first, slowest first.

Closes ticket 30860.
Closes ticket 31859 for 0.2.9.

Part of 31859.

An 80-character line (79 characters if you don't count the newline)
should not be truncated, and should not have a "..." insterted.

This allows the python doctest module to process it correctly
when invoked as:
   python -m doctest -v annotate_ifdef_directives.py

This algorithm is not fully general, but it strikes a balance
between efficiency, simplicity, and correctness.

Our line limit is 80 characters, assuming that there is a single
terminating newline character that counts towards the limit.  On
Windows, this might go as high as 81 characters, if we count CRLF as
two characters.

This change should reduce the number of cases where we say
"/* !(!defined(foo)) */" .

This only does cases where we can use a regex to make sure that the
simplification is guaranteed to be correct.  Full boolean
simplification would require this script to parse C, and nobody
wants that.

The jobs should be the same, but Travis may display them differently.

Part of 31859.

Fixes bug 31736; bugfix on 0.0.7.

The log mutex is dynamically initialized, guarded by log_mutex_initialized.
We don't want to destroy it, because after it is destroyed, we won't see
any more logs.

If tor is re-initialized, log_mutex_initialized will still be 1. So we
won't trigger any undefined behaviour by trying to re-initialize the
log mutex.

Part of 31736, but committed in this branch to avoid merge conflicts.

Part of 31614.

cb_buf_mutex is statically initialised, so we can not destroy it when
we are shutting down the err subsystem. If we destroy it, and then
re-initialise tor, all our backtraces will fail.

Part of 31736, but committed in this branch to avoid merge conflicts.