fix integer overflows in time_t/struct tm conversion code

fix integer overflows in time_t/struct tm conversion code

as found and reported by Brian Mastenbrook, the expressions
400*qc_cycles and years+100 in __secs_to_tm were both subject to
integer overflow for extreme values of the input t.

this patch by Szabolcs Nagy fixes the code by switching to larger
types, and matches the original intent I had in mind when writing this
code.