syscall: Dup3/Dup2/Dup should not use RawSyscall

[jacobsa]

These syscalls may block if there is a FUSE file system involved, causing them to camp on GOMAXPROCS credit. See this thread for details and discussion.

[jacobsa]

Just for the record, I see that you hit golang.org/x/sys/unix with golang/sys@5829c74, too. Thank you!