procd: add service instance watchdog

procd: add service instance watchdog

Added instance watchdog which will eventually either terminate
or respawn an instance depending on the instance respawn setting.

Added service ubus method 'watchdog' which services the watchdog
timer and allows update of the instance watchdog mode instance.

Two modes: disabled or passive.

Disabled: cancels watchdog timer set for a given instance.

Passive: sets a instance timer which must be serviced or the
instance will be stopped/restarted (dependent upon the instance
respawn value) when the timer expires.

Signed-off-by: Daniel Bailey <danielb@meshplusplus.com>