cloud backup: old iridium with working --headless for 7.6

"new headless" doesn't work in the chromium-based ports on BSDs[1] and
"old headless" mode was removed[2], so --headless doesn't work any more.
besides use with "web drivers" for testing/automation, this is also often
used for converting web pages to png/pdf:

iridium --disable-gpu --headless --screenshot=/tmp/a.png https://www.openbsd.org/
iridium --disable-gpu --headless --print-to-pdf=/tmp/a.pdf https://www.openbsd.org/faq/

in case anyone else needs this (and for me next time i forget which
machine i built it on, because i was already doing this for 7.5 and
obviously broke it after updating), here's a tar of an old version of
the iridium port (with a fix added to cope with the libxml API break)
that builds and runs ok on 7.6.

[1]
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275729
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278413
https://forums.freebsd.org/threads/chromium-headless-immediate-abort.91334/

[2]
https://developer.chrome.com/blog/chrome-headless-shell