illumos gate

A number of improvements have recently been made to bhyve's VNC support in FreeBSD including support for the native MacOS screen sharing client.
We should pull those into illumos.

https://reviews.freebsd.org/rS368747

Fix issues with various VNC clients.

* support VNC version 3.3 (macos "Screen Sharing" builtin client)
* wait until client has requested an update prior to sending framebuffer data
* don't send an update if no framebuffer updates detected
* increase framebuffer poll frequency to 30Hz, and double that when kbd/mouse input detected
* zero uninitialized array elements in rfb_send_server_init_msg()
* fix overly large allocation in rfb_init()
* use atomics for flags shared between input and output threads
* use #defines for constants