Add support for long names for registers

We can now access register / with the name slash, * with star,
and | with pipe

Fixes #23
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.
Do not touch the reference line in copy_indent
Merge branch 'master' into remove-buffer-change-listener
Refactor InputMode::Insert::prepare