This fixes several bugs in the code to set just the time (and

This fixes several bugs in the code to set just the time (and
not the date as well) on the RTC.  The low level function
x1205_set_datetime always wrote the date, even if it wasn't
given, resulting (typically) in the write of 0 bytes for the
mday,month and year.  It now only write h/m/s if the date is not
given.  The validate_tm API always checked the date, even if it
wasn't given (NOTE: the add of 'epoch' rather than 1900 may be
wrong, but it works at present).

The sync API fails to handle large resets, this problem still
exists, although the API is probably slightly more optimal.

BKrev: 4294039111Q_tc0c6nvKwVJbRiQY_w