Set close-on-exec flag on duplicate client and eventloop-registered FDs

Merge pull request #744 from panjf2000/dev

patch: v2.9.6

A few optimization, no bugfixes.

Merge pull request #730 from panjf2000/dev

patch: v2.9.4

Merge pull request #725 from panjf2000/dev

patch: v2.9.3

Merge pull request #719 from panjf2000/dev

patch: v2.9.2

bug: make Conn.Write/Writev return the non-retryable error if any (#715…

…) (#716)

Fixes #714

Merge pull request #710 from panjf2000/dev

minor: v2.9.0

Merge pull request #706 from panjf2000/dev

minor: v2.8.0

Merge pull request #682 from panjf2000/dev

patch: v2.7.2