Fix overflow in Multi::timeout_i32

[rizwan3659]

timeout_i32 has an off-by-one in its clamp check. It compares the whole
seconds against i32::MAX / 1000 using >, so a duration of exactly
2147483 seconds skips the clamp and goes into the arithmetic branch. If the
sub-second part is 648ms or more, secs * 1000 + millis ends up past
i32::MAX.

In debug builds this panics with "attempt to add with overflow". In release
builds it wraps around to a negative number instead, and libcurl's
multi_wait() rejects negative timeouts, so Multi::wait() and
Multi::poll() return CURLM_BAD_FUNCTION_ARGUMENT instead of waiting.

This changes it to work out the timeout in milliseconds first and then clamp
it to i32::MAX, so there's no intermediate value that can overflow. I also
added a unit test for the boundary values and for Duration::MAX.

Fixes #659