rclone: Rework backend option parsing

[fd0]

What is the purpose of this change? What does it change?

This change allows passing no arguments to rclone, using -o rclone.args="". It is helpful when running rclone remotely via SSH
using a key with a forced command (via command= in authorized_keys).

Was the change discussed in an issue or in the forum before?

On IRC

Checklist

• I have read the Contribution Guidelines
• I have added tests for all changes in this PR
• I have added documentation for the changes (in the manual)
• [-] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review

[codecov-io]

Codecov Report

Merging #2193 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2193      +/-   ##
==========================================
+ Coverage   50.76%   50.77%   +0.01%     
==========================================
  Files         176      176              
  Lines       14251    14244       -7     
==========================================
- Hits         7234     7232       -2     
+ Misses       5964     5958       -6     
- Partials     1053     1054       +1

Impacted Files	Coverage Δ
internal/backend/rclone/backend.go	66.66% <ø> (+2.33%)	⬆️
internal/backend/rclone/config.go	81.81% <100%> (-1.52%)	⬇️
internal/archiver/file_saver.go	85.96% <0%> (-2.64%)	⬇️
internal/archiver/blob_saver.go	97.53% <0%> (+2.46%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b8d1dc...3865b59. Read the comment docs.