Fix macOS fcntl/fsync usage properly

[ychin]

When fcntl fails, don't check errno when deciding whether to call fsync, as there are different possible errno results and they are not documented in the man page. Instead, always fall back to fsync. Worst case is that it's only slightly slower but that should only happen when something is wrong anyway so that's not a big deal.

Also see: #4016 for the first attempt to fix this.

[ychin]

Per my response on the vim_mac mailing list (https://groups.google.com/forum/#!topic/vim_mac/LVjNiPjhnxc):

Hi Bram, you have already merged my pull request, but I looked at the corresponding commit in Neovim that got spawned from this change, and it referenced this commit in libuv that has more details: libuv/libuv#2135

From that commit I think we are currently missing a case for webdav because we are not handling the case where it sets errno to "EINVAL". I think we should probably just do what that commit does and ignore errno altogether. The worst case is it will be slightly slower but that's only if something is wrong anyway. Otherwise it will be whack-a-mole with random undocumented errno's that are probably not intended to be used this way. I would think Apple just assumes the programmer fall back to fsync if fcntl failed for whatever reason.

[codecov-io]

Codecov Report

Merging #4025 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4025      +/-   ##
==========================================
- Coverage   79.15%   79.14%   -0.02%     
==========================================
  Files         105      105              
  Lines      141126   141126              
==========================================
- Hits       111707   111689      -18     
- Misses      29419    29437      +18

Impacted Files	Coverage Δ
src/fileio.c	75.44% <ø> (ø)	⬆️
src/gui_beval.c	61.57% <0%> (-0.88%)	⬇️
src/gui.c	58% <0%> (-0.57%)	⬇️
src/gui_gtk_x11.c	48.32% <0%> (-0.15%)	⬇️
src/window.c	83.49% <0%> (-0.14%)	⬇️
src/sign.c	93.38% <0%> (-0.13%)	⬇️
src/ui.c	50.14% <0%> (-0.08%)	⬇️
src/terminal.c	78.21% <0%> (-0.05%)	⬇️
src/screen.c	80.32% <0%> (+0.02%)	⬆️
src/channel.c	83.22% <0%> (+0.07%)	⬆️
... and 1 more

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