Fix DNS-SD sharing issues (Issue #1217)

Fix DNS-SD sharing issues (Issue #1217)

- Follow mDNS hostname changes unless explicitly configured with the
  DNSSDHostName directive.
- Add DNSSDComputerName directive to control the "@ computer name" suffix on
  share names.
- Update registrations whenever we detect a conflict or a hostname change.
- Add RWLock for Printers array so that we can iterate over the printers list
  in the registration callback (separate thread).
- Update cupsd.conf man page.