Add a UserInterface::is_ok method and return false on
SIGHUP/stdin closing/socket dropping
This should be cleaner and more robust than the previous SIGHUP
handling code.
Fixes#1594
This round trip through an input callback expected to call
is_key_available and get_key was overcomplicated, just send the
keys as they arrive, the client is already buffering due to urgent
event mode.
Add a ColumnCount type and use it in place of CharCount whenever
more appropriate, take column size of codepoints into account for
vertical movements and docstring wrapping.
Fixes#811
