inf-ptrace: Support async targets in inf_ptrace_target::wait.

- Handle TARGET_WNOHANG by passing WNOHANG to waitpid and returning
  TARGET_WAITKIND_IGNORE if there are no events to report.

- Handle a race in async mode where SIGCHLD might signal the event
  pipe for an event that has already been reported.  If the event was
  the exit of the last child process, waitpid() will fail with ECHILD
  rather than returning a pid of 0.  For this case, return
  TARGET_WAITKIND_NO_RESUMED.