Add FreeBSD support

For FreeBSD 12.2 and before I think we could use SCM_CREDS to get
the pid of the peer. For FreeBSD 13.x and later, the Xucred is
extended with a pid. For now we'll leave it unimplemented and
expect to add FreeBSD 13+ support whern it is released.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

peercred_freebsd.go

@@ -0,0 +1,55 @@
+// Copyright (c) 2021 AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package peercred
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+
+	"golang.org/x/sys/unix"
+)
+
+func init() {
+	osGet = getFreeBSD
+}
+
+func getFreeBSD(c net.Conn) (*Creds, error) {
+	switch c := c.(type) {
+	case *net.UnixConn:
+		return getUnix(c)
+	case *net.TCPConn:
+		// TODO: use /proc tcp info for localhost connections like Windows?
+	}
+	return nil, ErrUnsupportedConnType
+}
+
+func getUnix(c *net.UnixConn) (*Creds, error) {
+	raw, err := c.SyscallConn()
+	if err != nil {
+		return nil, fmt.Errorf("SyscallConn: %w", err)
+	}
+
+	var cred *unix.Xucred
+	cerr := raw.Control(func(fd uintptr) {
+		cred, err = unix.GetsockoptXucred(int(fd),
+			unix.SOL_LOCAL,
+			unix.LOCAL_PEERCRED)
+		if err != nil {
+			err = fmt.Errorf("unix.GetsockoptXucred: %w", err)
+			return
+		}
+	})
+	if cerr != nil {
+		return nil, fmt.Errorf("raw.Control: %w", err)
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &Creds{
+		pid: 0, // FreeBSD 13 adds a cr_pid field, can be used here.
+		uid: strconv.FormatUint(uint64(cred.Uid), 10),
+	}, nil
+}