Make posix_fallocate robust against interruption, report right errors

[ironage]

man posix_fallocate states:

posix_fallocate() returns zero on success, or an error number on failure.  Note that errno is not set.

With this in mind, we should report the return value and not the value in errno.

Additionally, EINTR is reported as a return code in fedora 24 (but strangely not mentioned in ubuntu 16). If it is interrupted, we can retry. It is hard to guess what the result of an interrupted posix_fallocate call would result in if not retried; the file would not be the expected size so this may fix errors like #2258.

[realm-ci]

Check the performance result here: https://ci.realm.io/job/realm/job/realm-core/job/PR-2905/1/Performance_Report

[codecov-io]

Codecov Report

Merging #2905 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2905      +/-   ##
==========================================
+ Coverage   92.62%   92.62%   +<.01%     
==========================================
  Files         245      245              
  Lines       75787    75790       +3     
  Branches     4346     4347       +1     
==========================================
+ Hits        70194    70197       +3     
  Misses       5250     5250              
  Partials      343      343

Impacted Files	Coverage Δ
src/realm/util/file.cpp	76.11% <50%> (+0.11%)	⬆️
test/test_thread.cpp	81.35% <0%> (+0.03%)	⬆️

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[kspangsege]

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