threads: add vlc_once_begin() and vlc_once_complete()

This supports one-time initialisation without callback, which is somewhat more flexible and provides better type safety.

added Component::Core label

mentioned in merge request !1429 (merged)

added MRStatus::Reviewable label

changed milestone to %4.0

added MRStatus::InReview label and removed MRStatus::Reviewable label

resolved all threads

approved this merge request

added MRStatus::Acceptable label and removed MRStatus::InReview label

added MRStatus::Accepted label and removed MRStatus::Acceptable label

MR Acceptance result

This MergeRequest has been Accepted! Congratulations.

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added 23 commits

• 0da095bd...dc26eb53 - 21 commits from branch videolan:master
• c86e6039 - threads: add vlc_once_begin() and vlc_once_complete()
• 8b4fec24 - threads: rewrite and inline vlc_once()

Compare with previous version

enabled an automatic merge when the pipeline for 8b4fec24 succeeds

merged