[WIP] Upstream ZESTI socket model

[kren1]

This pull request tries to upstream ZESTI socket model by @paulmar . It still needs some work, but I would appreacite any comments:

• Write more tests, but I ran out of ideas for them, so any suggestion welcome!
• Merge in klee-uclibc patch that removes building of some socket related objects (currently I just changed travis to my branch, so it can run)
• Currently this treats all socket connections as symbolic, so you can't have concrete socket connections. It's a bit difficult to decide when to treat a socket as symbolic and concrete. In ZESTI, it seemed to be controlled with, whether it's run in "ZESTI mode" or not. The way files are handled (with A,B,C file names) is not really helpful in this context, because you need to decide if a socket is symbolic or not at socket() call, where the address is still unknown.

[codecov]

Codecov Report

Merging #886 into master will decrease coverage by 0.11%.
The diff coverage is 30.18%.

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
- Coverage    65.9%   65.78%   -0.12%     
==========================================
  Files         141      141              
  Lines       15760    15808      +48     
  Branches     3701     3711      +10     
==========================================
+ Hits        10386    10399      +13     
- Misses       3704     3732      +28     
- Partials     1670     1677       +7

Flag	Coverage Δ
#systemtests	65.25% <30.18%> (-0.12%)	⬇️
#unittests	9.64% <0%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
runtime/POSIX/klee_init_env.c	37.19% <17.64%> (-3.77%)	⬇️
runtime/POSIX/fd_init.c	56.43% <36.11%> (-10.24%)	⬇️

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[kren1]

I don't think this is merge-able so I'm going to close. It is a nice starting point if anyone wants to model sockets though.