Simplify code and fix test

[zhsj]

These are some clean up, so I just send in one PR. And I add a go.mod file.

[codecov[bot]]

Codecov Report

Merging #65 into master will not change coverage by %. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   15.89%   15.89%           
=======================================
  Files          10       10           
  Lines         258      258           
=======================================
  Hits           41       41           
  Misses        212      212           
  Partials        5        5           

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[zhsj]

I didn't realize go1.3 is still in travis config. So I reverted the code to use unix.Dup2 directly.