Closes 31923.

There is a bad design choice in two of our configuration types,
where the empty string encodes a value that is not the same as the
default value.  This design choice, plus an implementation mistake,
meant that config_dup() did not preserve the value of routerset_t,
and thereby caused bug #31495.

This comment-only patch documents the two types with the problem,
and suggests that implementors try to avoid it in the future.

Closes ticket 31907.

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.

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.