[$] Bringing restartable sequences out of the niche

The restartable sequences feature, whichwas added to the 4.18 kernel in 2018, exists to enable better performancein certain types of threaded applications. While there are users forrestartable sequences, they tend to be relatively specialized code; this isnot a tool that most application developers reach for. Over time, though,the use of restartable sequences has grown, and it looks to grow further asthe feature is tied to new capabilities provided by the kernel. Asrestartable sequences become less of a niche feature, though, some problemshave turned up; fixing one of them may involve an ABI change visible inuser space.