iface-modem-signal: polling and thresholds only in effect if modem enabled
If the modem is disabled:
- Polling is completely halted.
- Thresholds are disabled.
- The user is allowed to call Setup() or SetupThresholds() to change the settings, even if the actual polling or thresholds setup isn't in effect.
When the modem is enabled:
- Polling will be started if there is a existing polling rate.
- Thresholds will be setup based on the existing threshold settings.
Fixes #504 (closed)