fuse: Correct behavior for reading after EOF, add snapshot template string

[fd0]

This PR corrects the behavior when data is read after the end of a file, and adds a CLI option to specify the template for the snapshot directories.

[codecov-io]

Codecov Report

Merging #1554 into master will increase coverage by 0.19%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #1554      +/-   ##
==========================================
+ Coverage   51.55%   51.74%   +0.19%     
==========================================
  Files         138      138              
  Lines       11081    11088       +7     
==========================================
+ Hits         5713     5738      +25     
+ Misses       4503     4480      -23     
- Partials      865      870       +5

Impacted Files	Coverage Δ
internal/fuse/root.go	0% <ø> (ø)	⬆️
internal/fuse/dir.go	0% <ø> (ø)	⬆️
internal/fuse/snapshots_dir.go	0% <0%> (ø)	⬆️
internal/fuse/file.go	67.01% <0%> (-0.7%)	⬇️
cmd/restic/cmd_mount.go	57.14% <60%> (-2.02%)	⬇️
internal/backend/test/tests.go	58.8% <0%> (+0.77%)	⬆️
internal/backend/s3/s3.go	65.76% <0%> (+1.15%)	⬆️
internal/checker/checker.go	71.33% <0%> (+3.77%)	⬆️

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