Replace calls to Fd() with syscall.RawConn.Control()

[robertodauria]

Fd() removes the O_NONBLOCK flag from the descriptor before returning it.

From os/file_unix.go:

	// If we put the file descriptor into nonblocking mode,
	// then set it to blocking mode before we return it,
	// because historically we have always returned a descriptor
	// opened in blocking mode. The File will continue to work,
	// but any blocking operation will tie up a thread.

When Fd() is called on a *os.File obtained via conn.File(), this descriptor is actually a duplicate but the underlying socket is shared. The end result is that the original conn still thinks it has a non-blocking socket, while the socket actually became blocking. This causes things like conn.Close() blocking forever if no data is received.

This should actually resolve #361 .

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1601

• 39 of 53 (73.58%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 81.587%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tcpinfox/tcpinfox_linux.go	15	19	78.95%
bbr/bbr_linux.go	24	34	70.59%

Totals
Change from base Build 1597:	-0.3%
Covered Lines:	1799
Relevant Lines:	2205

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💛 - Coveralls

[stephen-soltesz]

- Thank you!

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained