MFC r327756-r327758

r327756:
Bugfix: on RTC chips with a 32-bit binary counter, after reading the time,
return immediately rather than falling through to the logic that reads
BCD-encoded time.

r327757:
Bugfix: don't lose the am/pm mode flag when setting the time. Unlike most
RTC chips that have a control register bit for am/pm mode, the DS13xx series
uses one of the high bits in the hour register. Thus, when setting the time
in am/pm mode, the am/pm mode flag has to be ORed into the hour.

r327758:
Convert a collection of unrelated bitwise flags to a collection of boolean
vars in the softc. It makes the code more compact and readable, and
actually uses less memory too.