syscall: remove/update outdated TODO comments

syscall: remove/update outdated TODO comments

Error returns for linux/arm syscalls are handled since a long time.

Remove another list of unimplemented syscalls, following CL 96315.

The root-only check in TestSyscallNoError was shown to be sufficient as
part of CL 84485 already.

NetBSD and OpenBSD do not implement the sendfile syscall (yet), so add a
link to golang.org/issue/5847

Change-Id: I07efc3c3203537a4142707385f31b59dc0ecca42
Reviewed-on: https://go-review.googlesource.com/97115
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>