Instead of having them dump into the trash and initialize it, let's have
the caller initialize a buffer and pass it. This will be convenient to
dump multiple threads at once into a single buffer.
The new function ha_thread_dump() will dump debugging info about all known
threads. The current thread will contain a bit more info. The long-term goal
is to make it possible to use it in signal handlers to improve the accuracy
of some dumps.
The function dumps its output into the trash so as it was trivial to add,
a new "show threads" command appeared on the CLI.
