basic/escape: allow truncation mode where "…" is always appended

basic/escape: allow truncation mode where "…" is always appended

So far we would append "…" or "..." when the string was wider than the specified
output width. But let's add a mode where the caller knows that the string being
passed is already truncated.

The condition for jumping back in utf8_escape_non_printable_full() was
off-by-one. But we only jumped to that label after doing a check with a
stronger condition, so I think it didn't matter. Now it matters because we'd
output the forced ellipsis one column too early.