monitor: fix -MD/-M difference in property output
Fixes #241. * spot/twaalgos/postproc.cc: Use the deterministic monitor if it has as many states as the non-deterministic one. * spot/twaalgos/minimize.cc (minimize_monitor): Quickly check for terminal automata. * spot/twaalgos/stripacc.cc: Set the weak property. * spot/twaalgos/stripacc.hh: Improve documentation. * tests/core/monitor.test, tests/core/sbacc.test: Update. * NEWS: Mention the issue.