opts: parseKeyValueFile: cleanup and remove redundant trimming

[thaJeztah]

• relates to introduce ParseKeyValueFile #5491

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• the function already trimmed leading whitespace from each line before parsing. keys with trailing whitespace would be invalidated, and values have whitespace preserved, so there's no need to trim whitespace for the key.
• if a line is validated (key is valid), we don't need to reconstruct the key=value by concatenating, and we can add the line as-is.
• check if the key is empty before checking if it contains whitespace
• touch-up comments
• rename some variables for readability
• slight cleanup to use early returns / early continues to reduce nesting

[thaJeztah]

I'll rebase #5491 after this; still looking at separating this to a separate package, so there will be some follow-ups.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.09%. Comparing base (dac7319) to head (76196db).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5496      +/-   ##
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- Coverage   60.09%   60.09%   -0.01%     
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  Files         345      345              
  Lines       23445    23441       -4     
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- Hits        14090    14086       -4     
  Misses       8381     8381              
  Partials      974      974              

[robmry]

LGTM (plus bikeshedding!).

[robmry]

It seems odd to search for whitespace like this, but trim leading whitespace with TrimLeftFunc(_, unicode.IsSpace) above?

strings.ContainsFunc(key, unicode.IsSpace) would catch things that were missed before (like non-breaking space), so I guess it'd be a breaking change. Maybe worthwhile though?

[thaJeztah]

Yeah, I was looking at that as well; indeed also considering whether we should change. I'll keep this one for a follow-up in the meantime, but good suggestion.