linux: simplify uv__dup2_cloexec()
linux: simplify uv__dup2_cloexec()

Assume the presence of the dup3() system call on Linux. It was added
in 2.6.27 and our baseline is 2.6.32.

PR-URL: https://github.com/libuv/libuv/pull/2665
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso