x/sys/unix: define new RTF flags introduced by macOS 11.3

[riraccuia]

@kumpfdp and I noticed there is a bunch of new routing flags that are being actively used starting with macOS 11.3

The sdk files don't seem to be out yet (or i just can't find it) at opensource.apple.com but here's one unofficial link: https://github.com/plentico/osxcross-target/blob/master/SDK/MacOSX11.3.sdk/usr/include/net/route.h

#define RTF_PROXY       0x8000000       /* proxying, no interface scope */
#define RTF_ROUTER      0x10000000      /* host is a router */
#define RTF_DEAD        0x20000000      /* Route entry is being freed */
#define RTF_GLOBAL      0x40000000      /* route to destination of the global internet */

[tklauser]

Please note that the syscall package is frozen per the package level godoc. These new constants should be added to the golang.org/x/sys/unix package instead.

[gopherbot]

Change https://golang.org/cl/343509 mentions this issue: unix: add RTF_GLOBAL on darwin

[tklauser]

RTF_PROXY, RTF_ROUTER and RTF_DEAD are already present in the latest version of golang.org/x/sys. I've sent https://golang.org/cl/343509 to add the missing RTF_GLOBAL.