WIP: new periodic

This PR should eventually become an enhanced version of the periodic framework
- configurable support for 16 bit timer/counter unit 1 or unit 3 present in many controllers
- additional "milliticks" with higher resolution than 50 Hz
- milliticks available to other modules using timer()-style hooks into the ISR (addresses issue #199)
- base module for an enhanced scheduler 

1st step: replace 50 Hz CTC-mode timer, basic milliticks support. Several more steps are necessary to make CPU_CLOCK_SUPPORT, NTP, DCF, etc. working again.

This is Work-In-Progress. Please review, test and comment!
I would temporarily disable the config option for FREQCOUNT_SUPPORT.