expand_set_arguments: defer set load for depgraph

This allows the depgraph to display the "Calculating dependencies"
message before sets are loaded, as discussed in bug #461412.
iter_owners: event loop for spinner, bug #461412
expand_set_arguments: fix expansion breakage

This broke in commit 16e1f2185bb7ed36d3119b29a50ca0ff75fa8b71.
Use writemsg for unicode safety in set messages.
prepstrip: fix readelf Build ID for bug #460510
prepstrip: strip .note.gnu.gold-version sections
prepstrip: skip redundant build ID ln -s calls