HAL_PWR_EnterSTANDBY() didn't; replace with HAL_PWR_EnterTrueSTANDBY()

[terrillmoore]

HAL_PWR_EnterSTANDBY(), due to bug #83, never entered standby; instead it did a sleep. For various reasons, our old code relied on the old behavior. To keep from breaking the old code, we need to change HAL_PWR_EnterSTANDBY() to explicitly enter STOP mode; and add a new HAL_PWR_EnterTrueSTANDBY() that does the right thing. Otherwise, code that used to sleep will now reboot on wakeup.