[mythtv] #10305 heads up, mysql.txt removal + config.xml changes + UPnP fixes

[Daniel Kristjansson]

I've attached the latest patchset to the ticket. I just wanted to
give everyone a heads up in case they had strong opinions on the
config.xml format or want to look at the UPnP backend discovery
changes.

Summary of changes:
* Format of config.xml
 + Database settings have their own top-level instead of being under
   <MythFrontend><DefaultBackend>.
 + Wake On Lan settings have been added to config.xml, these
   were formerly only settable using mysql.txt.
* UPnP autoconf now waits a full 2 seconds before deciding there
  really is only one backend to connect to.
* UPnP resends the discovery packet a few times in auto conf mode.
* config.xml is only rewritten when it changes, and this is done
  in a safe manner so a crash will not wipe out your configuration.
* mysql.txt reading has been removed. This does not port over
  Wake On Lan settings and all other settings not written to the
  file by default are ignored. Setting file functionality is
  available using the --override-settings-file option, which
  has the added benefit of overriding DB settings instead of just
  providing new defaults if the DB doesn't have that setting.

I'll commit tomorrow if there are no objections.

-- Daniel