SE-0088: Modernize libdispatch for Swift 3 naming conventions

All for it!

···

I believe that the “Recv” on the right is a typo here:

DISPATCH_SOURCE_TYPE_MACH_RECV DispatchSourceMachRecv
since it’s spelled out in other places:

    class func machReceive(port: mach_port_t, queue: DispatchQueue? = nil) -> DispatchSourceMachReceive

(If I’m wrong then I object to this one abbreviation. :)

-Wil