treewide: replace /^I: / prefix with /^# /

treewide: replace /^I: / prefix with /^# /

This is like more familiar to readers of TAP (Test Anywhere
Protocol) output, as well as shell and Perl scripters which also
use `#' for comments.

AFAIK, nobody is parsing our stderr, and I'm not sure how
standardized the `I:' prefix is (nor `W:' and `E:' are).  It's
already the prevailing style in Lei* code, too, so things have
been moving in that direction for a bit.