mythbuntu should use respawn limit in upstart script

[Thomas Mashos]

Mythbackend shouldn't auto-respawn forever. It should use the respawn limit. This needs to be tested in Natty

From http://upstart.ubuntu.com/wiki/Stanzas
respawn limit

Syntax: respawn limit count timeout
Example:
respawn limit 15 5
Description: This configures the respawn limits, which only apply if the respawn flag is set. The respawn limits function as follows: If the process is respawned more than count times within an interval of timeout seconds, the process will be stopped automatically, and not restarted. Unless set explicitly, the limit defaults to 10 times within 5 seconds.
A separate respawn stanza must be present to actually set the respawn flag, as of version 0.3.9.