system: add reboot method to system ubus object

system: add reboot method to system ubus object

Sometimes, for various reasons, user may want to reboot a device. This
is a common task and it makes sense to support it with something common
like a procd.

Right now both: LuCI and LuCI2 implement this feature on their own with
luci-rpc-luci2-system reboot and luci-rpc-sys reboot. This leads to code
duplication and situation may become even worse with more software
controlling system with ubus.

Othen than that procd already has support for rebooting so one may
consider this ubus method even cleaner.

Once we get this patch in place we may consider switching LuCI and LuCI2
to this new method.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>