libproc-rs-43 Implement kmsgbuf() for linux

[andrewdavidmackenzie]

• make dmesg.rs cross-platform relying on the library implementing kmsgbuf() on linux
• kmesg_buffer - make all macos code only compile for mac and a skeleton function for linux
• mod.rs always include kmesg_buffer.rs

[codecov[bot]]

Codecov Report

Merging #53 into master will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   92.70%   92.78%   +0.07%     
==========================================
  Files           4        4              
  Lines         288      291       +3     
==========================================
+ Hits          267      270       +3     
  Misses         21       21              

Impacted Files	Coverage Δ
src/libproc/helpers.rs	85.71% <100.00%> (ø)
src/libproc/kmesg_buffer.rs	100.00% <100.00%> (ø)
src/libproc/proc_pid.rs	94.04% <100.00%> (ø)

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