httpc: fix zero timeout handling

When libcurl is built with --enable-threaded-resolver (which is default)
and the version of the library is 7.60 or above, libcurl calls a timer
callback with exponentially increasing timeout_ms value during DNS
resolving.

This behaviour was introduced in curl-7_59_0-36-g67636222f (see [1],
[2]). During first ten milliseconds the library sets a timer to a passed
time divided by three (see Curl_resolver_getsock()). It is possible that
passed time is zero during at least several thousands of iterations.

Before this commit we didn't set a libev timer in curl_multi_timer_cb()
when a timeout_ms value is zero, but call curl_multi_process()
immediately. Libcurl however can call curl_multi_timer_cb() again and
here we're going into a recursion that stops only when timeous_ms
becomes positive. Often we generate several thousands of stack frames
within this recursion and exceed 512KiB of a fiber stack size.

The fix is easy: set a libev timer to call curl_multi_process() even
when a timeout_ms value is zero.

The reason why we did the call to curl_multi_process() immediately is
the unclear wording in the CURLMOPT_TIMERFUNCTION option documentation.
This documentation page was fixed in curl-7_64_0-88-g47e540df8 (see [3],
[4], [5]).

There is also the related change in curl-7_60_0-121-g3ef67c686 (see [6],
[7]): after this commit libcurl calls a timer callback with zero
timeout_ms during a first three milliseconds of asynchronous DNS
resolving.

Fixes #4179.

[1]: https://github.com/curl/curl/pull/2419
[2]: https://github.com/curl/curl/commit/67636222f42b7db146b963deb577a981b4fcdfa2
[3]: https://github.com/curl/curl/issues/3537
[4]: https://github.com/curl/curl/pull/3601
[5]: https://github.com/curl/curl/commit/47e540df8f32c8f7298ab1bc96b0087b5738c257
[6]: https://github.com/curl/curl/pull/2685
[7]: https://github.com/curl/curl/commit/3ef67c6861c9d6236a4339d3446a444767598a58