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
  
  This work was contributed by Marko Kiiskila, with reuse of some earlier

work by Henrik Gulbrandsen.

Clients tested :
FreeBSD-current

• tightvnc
• tigervnc
• krdc
• vinagre

Linux (Ubuntu)

• krdc
• vinagre
• tigervnc
• xtightvncviewer
• remmina

MacOS

• VNC Viewer
• TigerVNC
• Screen Sharing (builtin client)

Windows 10

• Tiger VNC
• VNC Viewer (cursor lag)
• UltraVNC (cursor lag)

o/s independent

• noVNC (browser) using websockify relay

PR: 250795
Submitted by: Marko Kiiskila <marko@apache.org>
Reviewed by: jhb (bhyve)
MFC after: 3 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27605