unix: add SysctlClockinfo on OpenBSD

OpenBSD (like NetBSD) uses sysctl with struct clockinfo to get clock
rate information from the kernel. Add type Clockinfo and the
SysctlClockinfo function to query this information.

Change-Id: I35070a82b8de23dcd7592e8654dcc5eeee143b5b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/168057
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_openbsd.go

@@ -43,6 +43,23 @@ func nametomib(name string) (mib []_C_int, err error) {
 	return nil, EINVAL
 }
 
+func SysctlClockinfo(name string) (*Clockinfo, error) {
+	mib, err := sysctlmib(name)
+	if err != nil {
+		return nil, err
+	}
+
+	n := uintptr(SizeofClockinfo)
+	var ci Clockinfo
+	if err := sysctl(mib, (*byte)(unsafe.Pointer(&ci)), &n, nil, 0); err != nil {
+		return nil, err
+	}
+	if n != SizeofClockinfo {
+		return nil, EIO
+	}
+	return &ci, nil
+}
+
 func SysctlUvmexp(name string) (*Uvmexp, error) {
 	mib, err := sysctlmib(name)
 	if err != nil {