gzopen(): use Libc.errno()

[alyst]

I was trying to open the non-existing file, which triggered /lib/x86_64-linux-gnu/libc.so: Incorrect ELF header.
When I replaced unsafe_load(cglobal((:errno, :libc), Cint)) with Libc.errno(), the issue got resolved, and I got the proper "Not such file or directory" error.

Fixes #106

[alyst]

CI failures don't look related

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e1206d5) 9.98% compared to head (559c9e8) 10.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage    9.98%   10.15%   +0.16%     
==========================================
  Files          17       17              
  Lines        2374     2374              
==========================================
+ Hits          237      241       +4     
+ Misses       2137     2133       -4     

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[alyst]

@ViralBShah or @sjkelly, could you pls review the PR?

[sjkelly]

Thanks!