Add -rtcdate option to set initial date and/or time for RTCs

[ajrhacker]

• Remove system_time class from emu.h and rebase it on util::arbitrary_datetime
• util/timeconv.cpp: Add struct tm converter and day_of_week and day_of_year functions to arbitrary_datetime

[ajrhacker]

I'm putting this out here as a draft PR because, while I think the concept is worthwhile, the code changes are a bit messier than one might hope for. In particular, system_time::customize could probably be done in several neater ways, though the approach I used does enforce the format more strictly than others might.

I also had a hard time coming up with a good example for the docs, and finally remembered one of my own drivers. A lot of systems with RTCs don't display the time quickly or easily enough.