Use poll() instead of select()

[lbartnik]

• on *nix systems use poll() instead of select(); select has limitation on FD value to 1024
• test-utf8 is skipped if R does not support mbcs
• add control pipe to verify that the subprocess has already reached called execve()

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 0.91%. The diff coverage is 94.59%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   77.55%   76.64%   -0.92%     
==========================================
  Files          12       12              
  Lines         655      638      -17     
==========================================
- Hits          508      489      -19     
- Misses        147      149       +2

Impacted Files	Coverage Δ
src/sub-linux.cc	64.25% <94.59%> (-3.21%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 9ab368a...8f2f9fd. Read the comment docs.