[$] Splicing out vmsplice()

The splice() and vmsplice() system calls are meant to improve performance for certain data-movement tasks by minimizing (or avoiding altogether) system calls and the copying of data. They also have a long history of security problems. The recent flood of LLM-discovered vulnerabilities has drawn attention, once again, to splice() and vmsplice(); as a result, they may end up being removed altogether.