do back/forward ports (38)

[ThomasWaldmann]

https://github.com/borgbackup/borg/issues?utf8=%E2%9C%93&q=label%3Abackport%2F1.1-maint
https://github.com/borgbackup/borg/issues?utf8=%E2%9C%93&q=label%3Aforwardport%2Fmaster

do 3 simple/small or 1 bigger/complex forward/backport from there.

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💰 there is a bounty for this

[ThomasWaldmann]

working on:

• backport docs: mention double --force in prune docs #5357 : docs: mention double --force in prune docs #5358
• fwd port vagrant: install gcc-7 on openindiana #5363 : vagrant: install gcc-7 on openindiana #5364
• backport upgrade bundled xxhash to 0.8.0, fixes #5362 #5365 : upgrade bundled xxhash to 0.7.4 #5366

[ThomasWaldmann]

all done.