All BSDs use the same implementation to get BootTime{,WithContext} and
Uptime{,WithContext} based on the kern.boottime sysctl. Move this
implementation to a separate host/host_bsd.go file shared by darwin,
freebsd and openbsd. Also use SysctlTimeval to get
the boot time directly as a type Timeval instead of manually
extracting it using package unsafe. It will also allow for easier reuse
to support package host on e.g. Dragonfly BSD or NetBSD.
This requires updating the golang.org/x/sys/unix dependency to the
latest revision.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
