use C.kCFAllocatorDefault instead of refZero

[nonsense]

This PR is removing refZero and using C.kCFAllocatorDefault instead, which according to golang/text@572a2b1 and https://developer.apple.com/documentation/corefoundation/kcfallocatordefault is defined as NULL.

Current version does not compile under go version go1.10.4 darwin/amd64 and throws:

vendor/github.com/rjeczalik/notify/watcher_fsevents_cgo.go:51: cannot use refZero (type *_Ctype_struct___CFAllocator) as type _Ctype_CFAllocatorRef in argument to func literal
vendor/github.com/rjeczalik/notify/watcher_fsevents_cgo.go:169: cannot use refZero (type *_Ctype_struct___CFAllocator) as type _Ctype_CFAllocatorRef in argument to func literal
vendor/github.com/rjeczalik/notify/watcher_fsevents_cgo.go:170: cannot use refZero (type *_Ctype_struct___CFAllocator) as type _Ctype_CFAllocatorRef in argument to func literal

See: https://travis-ci.org/ethereum/go-ethereum/jobs/421003255#L164

[imsodin]

Did you check if it works with Go1.11?

[nonsense]

@imsodin yes, I checked, and this PR works with Go1.11 and Go1.10.4.

Actually current master (4e54e7f) works fine with Go1.11, however it doesn't work with Go1.10.4.

[rjeczalik]

Thanks @nonsense for the PR! I did know about C.kCFAllocatorDefault.