On macos/bsd systems, uuidgen produces all uppercase output. On
Linux/GNU systems it produces all lowercase output. This leads to
problems when managing org files across both systems (see e.g.
https://lists.gnu.org/archive/html/emacs-orgmode/2019-07/msg00081.html).
This change adds a defadvice for `org-id-new` to always lowercase the
output of `uuidgen` so the behavior is consistent across platforms.
f7fa3f3a86