use percentages in check_available_memory

[quaggoth]

Resolves #105

[codecov-io]

Codecov Report

Merging #106 into master will increase coverage by 1.84%.
The diff coverage is 98.3%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   79.28%   81.12%   +1.84%     
==========================================
  Files          11       13       +2     
  Lines         502      551      +49     
==========================================
+ Hits          398      447      +49     
  Misses        104      104

Impacted Files	Coverage Δ
lib/pkg/memory/memory.go	100% <100%> (ø)
lib/pkg/memory/memorylinux.go	100% <100%> (ø)
lib/app/nagiosfoundation/check_available_memory.go	84% <90%> (ø)	⬆️

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