Try multiple MythTV executables when retrieving --version information

This causes Smolt to try, in order, mythutil, mythbackend, and
mythfrontend, when polling them for MythTV version information. This is
to correct a situation where users return "Unknown" data because they
were using 3rd party packages that did not provide the backend
executable.
(cherry picked from commit ba8a1608cfd0d73c782abbe26f33336e91e530d5)