lib: fix c-ares thread misuse

The `struct thread **ref` that the thread code takes is written to and
needs to stay valid over the lifetime of a thread.  This does not hold
up if thread pointers are directly put in a `vector` since adding items
to a `vector` may reallocate the entire array.  The thread code would
then write to a now-invalid `ref`, potentially corrupting entirely
unrelated data.

This should be extremely rare to trigger in practice since we only use
one c-ares channel, which will likely only ever use one fd, so the
vector is never resized.  That said, c-ares using only one fd is just
plain fragile luck.

Either way, fix this by creating a resolver_fd tracking struct, and
clean up the code while we're at it.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

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