Remove -env-var-params support in :def command

It is not used
Improve commands documentation
Safer implementation of signal handlers in ncurses.cc

On recent ncurses implementation on cygwin, the old method provoked
freezes. Avoid calling ncurses functions in signal handlers.

We still call an unsafe function (EventManager::force_signal)...
Tweak prompt completion behaviour

Always select the common prefix if we just updated the list of
completions. The previous behaviour was to ignore it if we had
it already typed. Do that only if it was already displayed.