lib: Add "open dfd iter handling noent" helper, port tree-wide

lib: Add "open dfd iter handling noent" helper, port tree-wide

Follow up to a previous patch that addressed a double-close; I
realized we already had a helper for doing "open dfd iter, do nothing
if we get ENOENT".  Raise it to libotuil, and port all consumers.

Closes: #863
Approved by: jlebon