os: remove incorrect detection of O_CLOEXEC flag on darwin

The below range loop will not stop when encountering
the first '.' character in a Darwin version string like "15.6.0".

for i = range osver {
   if osver[i] != '.' {
         continue
      }
   }
}

Therefore, the condition i > 2 was always satisfied and
supportsCloseOnExec was always set to true.

Since the minimum supported version of OSX for go is currently 10.8
and O_CLOEXEC is implemented from OSX 10.7 on the detection code
can be removed and support for O_CLOEXEC is always assumed to exist.

Change-Id: Idd10094d8385dd4adebc8d7a6d9e9a8f29455867
Reviewed-on: https://go-review.googlesource.com/37193Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/sys_darwin.go

@@ -4,28 +4,8 @@
 
 package os
 
-import "syscall"
-
 // supportsCloseOnExec reports whether the platform supports the
 // O_CLOEXEC flag.
-var supportsCloseOnExec bool
-
-func init() {
-	// Seems like kern.osreldate is veiled on latest OS X. We use
-	// kern.osrelease instead.
-	osver, err := syscall.Sysctl("kern.osrelease")
-	if err != nil {
-		return
-	}
-	var i int
-	for i = range osver {
-		if osver[i] != '.' {
-			continue
-		}
-	}
-	// The O_CLOEXEC flag was introduced in OS X 10.7 (Darwin
-	// 11.0.0). See http://support.apple.com/kb/HT1633.
-	if i > 2 || i == 2 && osver[0] >= '1' && osver[1] >= '1' {
-		supportsCloseOnExec = true
-	}
-}
+// The O_CLOEXEC flag was introduced in OS X 10.7 (Darwin 11.0.0).
+// See http://support.apple.com/kb/HT1633.
+const supportsCloseOnExec = true