unix: add Xucred, GetsockoptXucred on freebsd

Follow CL 292330 which added these on darwin.

Generated on FreeBSD 13.0BETA3

For golang/go#41659

Change-Id: I30eeef43c4f61a9449f3fe8b5cc0033f46dfe822
Reviewed-on: https://go-review.googlesource.com/c/sys/+/294989
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/zerrors_freebsd_amd64.go

@@ -998,6 +998,11 @@ const (
 	KERN_OSRELEASE                 = 0x2
 	KERN_OSTYPE                    = 0x1
 	KERN_VERSION                   = 0x4
+	LOCAL_CONNWAIT                 = 0x4
+	LOCAL_CREDS                    = 0x2
+	LOCAL_CREDS_PERSISTENT         = 0x3
+	LOCAL_PEERCRED                 = 0x1
+	LOCAL_VENDOR                   = 0x80000000
 	LOCK_EX                        = 0x2
 	LOCK_NB                        = 0x4
 	LOCK_SH                        = 0x1
@@ -1377,6 +1382,7 @@ const (
 	SOCK_RDM                       = 0x4
 	SOCK_SEQPACKET                 = 0x5
 	SOCK_STREAM                    = 0x1
+	SOL_LOCAL                      = 0x0
 	SOL_SOCKET                     = 0xffff
 	SOMAXCONN                      = 0x80
 	SO_ACCEPTCONN                  = 0x2