style: simplify writing of some code and remove some redundant constants

[jievince]

Describe what this PR does / why we need it

• Modify some code to follow the rule RULE006 - Return fast to indent less
• Reome some redundant constants to unify the code style

Does this pull request fix one issue?

NONE

[codecov-io]

Codecov Report

Merging #144 into master will increase coverage by 0.04%.
The diff coverage is 12.82%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   14.85%   14.89%   +0.04%     
==========================================
  Files          56       56              
  Lines        2781     2773       -8     
==========================================
  Hits          413      413              
+ Misses       2331     2323       -8     
  Partials       37       37

Impacted Files	Coverage Δ
exec/os/script_exit.go	0% <0%> (ø)	⬆️
exec/os/network_dns.go	0% <0%> (ø)	⬆️
exec/os/script_delay.go	0% <0%> (ø)	⬆️
exec/os/network_loss.go	0% <0%> (ø)	⬆️
exec/os/cpu.go	14.15% <0%> (+0.13%)	⬆️
exec/os/disk_burn.go	0% <0%> (ø)	⬆️
exec/os/network_drop.go	0% <0%> (ø)	⬆️
exec/os/bin/burncpu/burncpu.go	46.75% <50%> (ø)	⬆️

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