kevent64() should support 0x1 fflag for EVFILT_MACHPORT

[jceel]

fflag 0x1 is called DISPATCH_MACH_SEND_DEAD in libdispatch and doesn't seem to be documented anywhere else. It's used to wait for mach port to become dead name.

There are more undocumented flags, which include at least:

• DISPATCH_MACH_SEND_POSSIBLE
• DISPATCH_MACH_SEND_DELETED

[mattmacy]

@jceel: What clients actually rely on SEND_DEAD? I don't see this flag or anything that would correspond to 0x1 in the EVFILT_MACH handler in xnu.