unix: support Linux NFC Subsystem

This adds the constants and types needed for the Linux NFC Subsystem.
It also adds support for sockaddr_nfc and sockaddr_nfc_llcp to Connect.

The Linux NFC Subsystem is documented in:
https://www.kernel.org/doc/html/latest/networking/nfc.html.

Change-Id: Ic6a79afdba85e9b154ec46f191f27714b4b38a57
Reviewed-on: https://go-review.googlesource.com/c/sys/+/323551
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_mipsx.go

@@ -229,6 +229,10 @@ func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint32(length)
 }
 
+func (rsa *RawSockaddrNFCLLCP) SetServiceNameLen(length int) {
+	rsa.Service_name_len = uint32(length)
+}
+
 //sys	poll(fds *PollFd, nfds int, timeout int) (n int, err error)
 
 func Poll(fds []PollFd, timeout int) (n int, err error) {