fix: preserve arch in SetPrimPart

[tri-adam]

Re-factor SetPrimPart, extracting the partition of the new (and old, if applicable) system partition and updating only the Parttype field.

In addition to the unit tests, the fix can be exercised like so:

$ go run ./cmd/siftool/ list test/images/one-group.sif ------------------------------------------------------------------------------ ID |GROUP |LINK |SIF POSITION (start-end) |TYPE ------------------------------------------------------------------------------ 1 |1 |NONE |32768-32772 |FS (Raw/System/386) 2 |1 |NONE |36864-40960 |FS (Squashfs/*System/386) $ go run ./cmd/siftool/ setprim 1 test/images/one-group.sif $ go run ./cmd/siftool/ list test/images/one-group.sif ------------------------------------------------------------------------------ ID |GROUP |LINK |SIF POSITION (start-end) |TYPE ------------------------------------------------------------------------------ 1 |1 |NONE |32768-32772 |FS (Raw/*System/386) 2 |1 |NONE |36864-40960 |FS (Squashfs/System/386) 

Closes #285

[dtrudg]

LGTM