Avoid syscall.Syscall use on OpenBSD

Syscall numbers are not stable on OpenBSD, and hardcoding the msync
syscall number will break bbolt on future versions of OpenBSD.  Use
the libc wrapper provided by golang.org/x/sys/unix instead.

Signed-off-by: Josh Rickmar <jrick@zettaport.com>

bolt_openbsd.go

@@ -1,22 +1,11 @@
 package bbolt
 
 import (
-	"syscall"
+	"golang.org/x/sys/unix"
-	"unsafe"
-)
-
-const (
-	msAsync      = 1 << iota // perform asynchronous writes
-	msSync                   // perform synchronous writes
-	msInvalidate             // invalidate cached data
 )
 
 func msync(db *DB) error {
-	_, _, errno := syscall.Syscall(syscall.SYS_MSYNC, uintptr(unsafe.Pointer(db.data)), uintptr(db.datasz), msInvalidate)
+	return unix.Msync(db.data[:db.datasz], unix.MS_INVALIDATE)
-	if errno != 0 {
-		return errno
-	}
-	return nil
 }
 
 func fdatasync(db *DB) error {