[$] GDB and io_uring

A problem reported when attaching GDB to programs that use io_uring has led to a flurry of potentialsolutions, and one that was merged into Linux 5.12-rc5. The problemstemmed from a change made in the 5.12merge window tohow the threads used by io_uring were created, such that they became associatedwith the process using io_uring. Those "I/O threads" were treatedspecially in the kernel, but that led to the problem with GDB (and likely other ptrace()-usingprograms). The solution is to treat them like other threads because itturned out that trying to makethem special caused more problems than it solved.