bessctl: add init/cmd argument introspection

[melvinw]

This has been useful for a project I've been working on. Might be worth including a version upstream.

This change is backwards compatible with modules that elect to keep exposing their argument/init types as strings

[codecov]

Codecov Report

Merging #1012 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1012   +/-   ##
=======================================
  Coverage   55.96%   55.96%           
=======================================
  Files           9        9           
  Lines        1165     1165           
=======================================
  Hits          652      652           
  Misses        513      513           

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