x/sys/unix: No MNT_FORCE constant on darwin

[jacobsa]

On OS X, man 2 unmount shows a flag for unmount called MNT_FORCE. However, on my system, neither syscall nor golang.org/x/sys/unix contain a constant with that name:

% go version
go version go1.4.1 darwin/amd64

% godoc syscall | grep -i force
    PT_FORCEQUOTA                     = 0x1e
    For testing: clients can set this flag to force creation of IPv6 sockets

% godoc golang.org/x/sys/unix | grep -i force 
    PT_FORCEQUOTA                     = 0x1e
    For testing: clients can set this flag to force creation of IPv6 sockets

For what it's worth, my Linux machine does have that constant in golang.org/x/sys/unix (but not in syscall).

[gopherbot]

Change https://golang.org/cl/54210 mentions this issue: unix: add MNT_* constants on Darwin