consider using I32 instead of U32 for Time

[mwette]

F´ Version	3.5.0
Affected Component	Fw/Time

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Feature Description

Time uses U32 to encode seconds and microseconds. Consider using I32 for these. There will arise the need to subtract times.

Rationale

1. Consider the case of subtracting 2^32 from 0 seconds. You need 33 bits for that.
2. A subtraction needs to return a time-difference type. If you use I32, Time can be used.
3. I32 is still big enough to add or subtract two values of nanoseconds, if you ever change to that.
4. Changing U32 to I32 is unlikely to break any existing code (that does not subtract time).

If you look in POSIX implementations for struct timespec you'll see that usually long and not unsigned long is used. I guess it is for the reasons above.