Refactor README.md

Fix README.md's Markdown formatting and split it into separate files
based on their original www.karlrunge.com contents & locations.

Changes:
* Remove old/broken www.karlrunge.com links
* Move 'Chaining SSH's' section into 'How to use x11vnc'
* Remove 'Beta Testing'/changelog section
* Move FAQ section into doc/FAQ.md
* Move 'Misc. build problems' section into 'Building x11vnc'
* Move 'Sun Ray Notes' into doc/SUNRAY.md
* Move 'Notes on x11vnc SSL Certificates and Key Management' into
  doc/SSL.md
* Move 'Using Apache as an SSL Gateway to multiple x11vnc servers
  inside a firewall' into doc/SSL-PORTAL.md
* Move 'Enhanced TightVNC Viewer (SSVNC: SSL/SSH VNC viewer)' into
  doc/SSVNC.md
* Move x11vnc options dump into doc/OPTIONS.md

Fixes LibVNC#137