added openbsd implementations

[f41gh7]

#785

removed fadvise for openbsd, added freespace implemenation for openbsd

[codecov]

Codecov Report

Merging #790 into master will increase coverage by 0.41%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #790      +/-   ##
==========================================
+ Coverage   65.32%   65.74%   +0.41%     
==========================================
  Files         172      176       +4     
  Lines       20285    20939     +654     
==========================================
+ Hits        13251    13766     +515     
- Misses       5810     5930     +120     
- Partials     1224     1243      +19     

Impacted Files	Coverage Δ
lib/fs/fs.go	5.00% <0.00%> (+0.03%)	⬆️
lib/fs/fs_nix.go	0.00% <0.00%> (ø)
lib/mergeset/block_stream_reader.go	49.30% <0.00%> (-13.20%)	⬇️
app/vmalert/remotewrite/remotewrite.go	56.60% <0.00%> (-1.89%)	⬇️
app/vmselect/promql/aggr_incremental.go	87.98% <0.00%> (-1.56%)	⬇️
lib/mergeset/table.go	64.75% <0.00%> (-0.94%)	⬇️
lib/storage/index_db.go	69.29% <0.00%> (-0.06%)	⬇️
lib/cgroup/mem.go	0.00% <0.00%> (ø)
lib/cgroup/cpu.go	24.48% <0.00%> (ø)
... and 6 more

---

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[valyala]

LGTM