unix: add SysctlTimeval on *bsd and darwin

Change-Id: I18daa353d101e6636a732288c8276d9f68d25f90
Reviewed-on: https://go-review.googlesource.com/c/sys/+/244237
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_bsd.go

@@ -527,6 +527,23 @@ func SysctlClockinfo(name string) (*Clockinfo, error) {
 	return &ci, nil
 }
 
+func SysctlTimeval(name string) (*Timeval, error) {
+	mib, err := sysctlmib(name)
+	if err != nil {
+		return nil, err
+	}
+
+	var tv Timeval
+	n := uintptr(unsafe.Sizeof(tv))
+	if err := sysctl(mib, (*byte)(unsafe.Pointer(&tv)), &n, nil, 0); err != nil {
+		return nil, err
+	}
+	if n != unsafe.Sizeof(tv) {
+		return nil, EIO
+	}
+	return &tv, nil
+}
+
 //sys	utimes(path string, timeval *[2]Timeval) (err error)
 
 func Utimes(path string, tv []Timeval) error {