cache: OS-specific cache directories

[prattmic]

Note: I haven't had a chance to test this on Windows yet, and I don't have an OS X machine.

Windows, and to a lesser extent OS X, don't conform to XDG and have
their own preferred locations for caches.

On Windows, use %LOCALAPPDATA%/restic (i.e., ~/AppData/Local/restic). I
can't find authoritative documentation from Microsoft recommending
specifically which of %APPDATA%, %LOCALAPPDATA%, and %TEMP% should be
used for caches, but %LOCALAPPDATA% is where browsers store their
caches, so it seems like a good fit.

On OS X, use ~/Library/Caches/restic, which is recommended by the Apple
documentation. They do suggest using the application "bundle identifier"
as the base folder name, but restic doesn't have one, so I just used
"restic".

[codecov-io]

Codecov Report

Merging #1287 into master will decrease coverage by 5.64%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #1287      +/-   ##
=========================================
- Coverage   52.25%   46.6%   -5.65%     
=========================================
  Files         134     134              
  Lines       13016   13039      +23     
=========================================
- Hits         6801    6077     -724     
- Misses       5384    6186     +802     
+ Partials      831     776      -55

Impacted Files	Coverage Δ
internal/cache/cache.go	29.87% <0%> (ø)	⬆️
internal/cache/dir.go	0% <0%> (ø)	⬆️
internal/backend/b2/b2.go	0% <0%> (-79.93%)	⬇️
internal/backend/azure/azure.go	0% <0%> (-75.35%)	⬇️
internal/backend/swift/swift.go	0% <0%> (-73.9%)	⬇️
internal/backend/gs/gs.go	0% <0%> (-65.6%)	⬇️
internal/backend/swift/config.go	34.37% <0%> (-56.25%)	⬇️
internal/archiver/archiver.go	65.2% <0%> (-0.17%)	⬇️

---

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[dionorgua]

Just small note.

%LOCALAPPDATA% is probably correct choose. The real difference is that in corporate environment %APPDATA% will be roamed when user sign to different machine (Sign in on PC1, store something in %APPDATA%, logout. Go to PC2 and sign in with same domain account. %APPDATA% will be synchronized).

The only thing I'm wondering is that %LOCALAPPDATA%/restic is subject to backup by other apps. Ideally restic should explicitly tell system that this directory has no important data by creating special registry key in FilesNotToBackup tree: https://msdn.microsoft.com/en-us/library/windows/desktop/bb891959(v=vs.85).aspx

[prattmic]

I can report that this works swimmingly on my Windows machine.

@dionorgua good to know thanks. Setting that registry key would be clever, but I think that is outside the scope of this PR.

[fd0]

LGTM!

[rawtaz]

Confirmed working on macOS 10.12.6.

[fd0]

Thank you very much for the confirmation. This change is better done sooner rather than later, while the number of people affected (in terms of "have an unused cache lying around") is small.

Thanks everyone!