RTC 12h to 24h mode conversion is wrong for noon and midnight

[dillof]

The conversion in getrtc simply adds 12 if PM is set.

However, in 12h mode, midnight is represented as hour=12 pm=0, which is converted to 24h mode as 12.

Likewise, noon is represented as hour=12, pm=1, which is converted to 24.

So, a special case is needed if hour==12: Set hour to 0 if pm==0, otherwise leave unchanged.

setrtc has the same problem in reverse.

[mlund]

@dillof Would you be willing to include these cases in tests/test-time.c?
Would be helpful for fixing.