model/main: disable most printing by default, add verbosity

model/main: disable most printing by default, add verbosity

Adds a '-v' parameter for printing verbose info on a non-DEBUG build.
This effectively creates 3 classes of runs:

 - Default build, no -v: only prints info when discovering a buggy
   execution
 - Default build, -v: prints more verbose information, on all feasible
   executions (buggy or non-buggy)
 - Debug build: Built with 'make debug'. Most verbose; prints everything
   for every execution, including infeasible executions. Also prints
   DEBUG() prints (as usual).