Introduce thread-safe handling for complaints

Introduce thread-safe handling for complaints

This introduces a new class that can be used to make the "complaint"
code thread-safe.  Instantiating the class installs a new handler that
collects complaints, and then prints them all when the object is
destroyed.

This approach requires some locks.  I couldn't think of a better way
to handle this, though, because the I/O system is not thread-safe.

It seemed to me that only GDB developers are likely to enable
complaints, and because the complaint macro handle this case already
(before any locks are required), I reasoned that any performance
degradation that would result here would be fine.

As an aside about complaints -- are they useful at all?  I just ignore
them, myself, since mostly they seem to indicate compiler problems
that can't be solved in the GDB world anyway.  I'd personally prefer
them to be in a separate tool, like a hypothetical 'dwarflint'.