Fix: set FD_CLOEXEC flag to unix client fd

[gunli]

1. Are you opening this pull request for bug-fix, optimization or new feature?

This PR is for bug-fix.
Currently, the Unix client fd will leak to the child process, because we call unix.Dup(int(fd))--which will remove the FD_CLOEXEC flag--to duplicate the raw fd, and that is unsafe.

	var dupFD int
	e := rc.Control(func(fd uintptr) {
		dupFD, err = unix.Dup(int(fd))	
	})

2. Please describe how these code changes achieve your intention.

// EnrollContext is like Enroll but also accepts an empty interface ctx that can be obtained later via Conn.Context.
func (cli *Client) EnrollContext(c net.Conn, ctx any) (Conn, error) {
	// ......
	var dupFD int
	e := rc.Control(func(fd uintptr) {
		dupFD, err = unix.Dup(int(fd))
		// Set the socket to close-on-exec, so that the socket is closed when the process forks
		unix.CloseOnExec(dupFD)
	})
	// ...... 
}

3. Please link to the relevant issues (if any).

4. What documentation changes (if any) need to be made/updated because of this PR?

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.31%. Comparing base (a46e1e7) to head (969c4b4).
⚠️ Report is 2 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #743   +/-   ##
=======================================
  Coverage   84.30%   84.31%           
=======================================
  Files          22       22           
  Lines        2402     2403    +1     
=======================================
+ Hits         2025     2026    +1     
  Misses        257      257           
  Partials      120      120           

Flag	Coverage Δ
unittests	84.31% <100.00%> (+<0.01%)	⬆️

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[panjf2000]

LGTM