[regression] CTRL+C no longer terminates tasks

[MartinNowak]

The process keeps running when it's send SIGINT or SIGTERM.
This was introduced between 0.7.17 and 0.7.18.

import vibe.d;

shared static this()
{
    runTask({while (true) yield();});
}

[MartinNowak]

Seems to be related to the busy yield loop. I'd expect yield to query the st_term flag.