allow module .so's to be symbolic links

[nrdmao]

If libtool is used to build libraries, then come in the form of .so.0.0.1 for example and then the .so is linked to the version specific .so. With this commit, bessd can load such symlinks. I also added some
logging to add diagnostics on modules loaded and failure to load modules.

[codecov]

Codecov Report

Merging #605 into master will increase coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #605      +/-   ##
==========================================
+ Coverage   69.18%   69.19%   +<.01%     
==========================================
  Files         204      204              
  Lines       12984    12987       +3     
==========================================
+ Hits         8983     8986       +3     
  Misses       4001     4001

Impacted Files	Coverage Δ
core/bessd.cc	68.75% <75%> (-0.04%)	⬇️
core/modules/flowgen.cc	74.07% <0%> (+0.37%)	⬆️

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[changlan]

LGTM. Contributor License Agreement pending.

[sangjinhan]