net: harmonize the time units used for TCP keep-alive on DragonFly and other UNIX's by seconds

Follows up CL 542275

Fixes #65809

Change-Id: Iba01efb4ff0fbb7a67840875322f0338337ebb78
Reviewed-on: https://go-review.googlesource.com/c/go/+/565315
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Andy Pan <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: panjf2000

src/net/tcpconn_keepalive_dragonfly_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || freebsd || linux || netbsd
+//go:build aix || dragonfly || freebsd || linux || netbsd
 
 package net
 

src/net/tcpconn_keepalive_unix_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || freebsd || linux || netbsd
+//go:build aix || dragonfly || freebsd || linux || netbsd
 
 package net