Bugfixes for message buffer
The message buffer has been fixed. It no longer relies on knowing the
length of the meassge buffer.
The message history also now works, being kept in a separate array that
will allow access to previous messages while also being truncated any
time a message would cause the message history buffer to exceed
MAX_MESSAGE_BUFFER.
Additionally, a warning caused by an unreachable statement in
CursesIO.getcommand() has been fixed.
The keymap taken in by SpelunkIO.getcommand is no longer a global
variable, but a function parameter.
Signed-off-by: Philip Pavlick <philip@swash.link>