curl_multi_poll: a sister to curl_multi_wait() that waits more

Hi team,

Very soon after we introduced curl_multi_wait() I realized it had an awkward
behavior that makes it tricky to use at times. Today I wrote up an initial
proposal for a drop-in function replacement that probably is more what
application authors actually want and need - and some probably already thought
curl_multi_wait() worked like this.

I'm open for all and any feedback! Nothing in this is written in stone yet...

See the relatively small patch in PR 4163:

   https://github.com/curl/curl/pull/4163

DESCRIPTION

Repeatedly we see problems where using curl_multi_wait() is difficult or
just awkward because if it has no file descriptor to wait for
internally, it returns immediately and leaves it to the caller to wait
for a small amount of time in order to avoid occasional busy-looping.

This is often missed or misunderstood, leading to underperforming
applications.

This change introduces curl_multi_poll() as a replacement drop-in
function that accepts the exact same set of arguments. This function
works identically to curl_multi_wait() - EXCEPT - for the case when
there's nothing to wait for internally, as then this function will by
itself wait for a "suitable" short time before it returns. This
effectiely avoids all risks of busy-looping and should also make it less
likely that apps "over-wait".

This also changes the curl tool to use this funtion internally when
doing parallel transfers and changes curl_easy_perform() to use it
internally.

-- 
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