executor: add some USB related TODOs

[xairy]

Also simplify timeout related code a bit.

[codecov-io]

Codecov Report

Merging #1319 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1319      +/-   ##
=========================================
- Coverage   57.66%   57.6%   -0.06%     
=========================================
  Files         131     131              
  Lines       24661   24661              
=========================================
- Hits        14221   14207      -14     
- Misses       9765    9780      +15     
+ Partials      675     674       -1

Impacted Files	Coverage Δ
pkg/csource/csource.go	76.31% <0%> (-5.85%)	⬇️
prog/rand.go	85.14% <0%> (+0.2%)	⬆️
prog/mutation.go	93.89% <0%> (+1.13%)	⬆️

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