support SIGINFO on Darwin

[Habbie]

Some OSes, like Darwin and FreeBSD, support a keyboard shortcut (usually ^T, configured via stty) that sends SIGINFO to the running process. Tools like dd, which on Linux give stats when sent SIGUSR1, will give out those stats on receipt of SIGINFO on Darwin and FreeBSD.

This PR adds support for SIGINFO on Darwin to restic. I hope it is correct - it appears the updateProgress channel is not at all reliable in causing a progress update to appear.

This is the first Go I've typed in years, so it is possible I went at this the wrong way. I checked with the #go-nuts IRC channel and I got the impression that build tags are the only way to accomplish this. I do not actually like the way this turned out, so please let me know if I've missed something.

[codecov-io]

Codecov Report

Merging #1082 into master will decrease coverage by 3.54%.
The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master    #1082      +/-   ##
==========================================
- Coverage   53.77%   50.22%   -3.55%     
==========================================
  Files         117      118       +1     
  Lines        8925     8933       +8     
==========================================
- Hits         4799     4487     -312     
- Misses       3420     3767     +347     
+ Partials      706      679      -27

Impacted Files	Coverage Δ
src/restic/progress_unix.go	57.14% <ø> (ø)	⬆️
src/restic/progress_unix_with_siginfo.go	62.5% <62.5%> (ø)
src/restic/backend/b2/b2.go	0% <0%> (-80.59%)	⬇️
src/restic/backend/swift/swift.go	0% <0%> (-73.69%)	⬇️
src/restic/backend/swift/config.go	36.95% <0%> (-54.35%)	⬇️

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[fd0]

I don't really like the code duplication either, but it's not worth it building a fancy abstraction for such a small detail. Thanks for your contribution!

[valpackett]

uhh… you mentioned "Darwin and FreeBSD", that's even mentioned in the release notes (!) but the actual source only mentions darwin!!

This should be enabled on all BSD systems.

[fd0]

Hm, I think you're right. Would you like to try a PR to add this? :)

[valpackett]

I thought that's too trivial for a PR but sure

[Habbie]

Yes, sorry, I did not have a FreeBSD box handy when doing the patch.