watchdog: Add watchdog pretimeout support

Add support for managing and configuring watchdog pretimeout values if
the watchdog hardware supports it. The ping interval is adjusted to
account for a pretimeout so that it will still ping at half the timeout
interval before a pretimeout event would be triggered. By default the
pretimeout defaults to 0s or disabled.

The RuntimeWatchdogPreSec config option is added to allow the pretimeout
to be specified (similar to RuntimeWatchdogSec). The
RuntimeWatchdogPreUSec dbus property is added to override the pretimeout
value at runtime (similar to RuntimeWatchdogUSec). Setting the
pretimeout to 0s will disable the pretimeout.