implement unix domain (ipc) socket support

[ThomasWaldmann]

borg serve  --socket=/path/to/repo/socket      # server (listening) side
borg --repo socket:///path/to/repo create ...  # client side

fixes #6183.

[ThomasWaldmann]

TODO / issues:

• server side log output does not go over the socket (socket is bidi, but just 2 channels total. ssh is 3 channels and has separate stderr).
• --socket wants the socket file path, socket:// URL wants the repo directory path. "socket" must be in repo directory.
• add more tests

[codecov-commenter]

Codecov Report

Merging #7519 (5d46220) into master (1e1c922) will decrease coverage by 0.15%.
The diff coverage is 48.14%.

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@@            Coverage Diff             @@
##           master    #7519      +/-   ##
==========================================
- Coverage   83.87%   83.72%   -0.15%     
==========================================
  Files          67       67              
  Lines       11817    11853      +36     
  Branches     2155     2162       +7     
==========================================
+ Hits         9911     9924      +13     
- Misses       1332     1352      +20     
- Partials      574      577       +3     

Impacted Files	Coverage Δ
src/borg/remote.py	78.01% <36.36%> (-2.94%)	⬇️
src/borg/archiver/_common.py	89.72% <100.00%> (ø)
src/borg/archiver/serve_cmd.py	100.00% <100.00%> (ø)
src/borg/helpers/parseformat.py	90.82% <100.00%> (+0.08%)	⬆️

... and 1 file with indirect coverage changes

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[ThomasWaldmann]

see #7615