code128: reduce extended latch cut-off from 5 to 4

[gitlost]

Latch/unlatch extended on 4 chars instead of 5 for better encodation in certain cases (and no pessimizations found so far), part addresses PR #272, props lyngklip

Also fix masked extended chars matching ASCII digits in numsscr (skipped using re-synced numEA array)

This is the first of two alternative PRs, the second one being an implementation of Alex Geller's divide & conquer algorithm as used by ZXing (and Zint)