Add cooked (ICANON) console input.

Add support for ICANON mode a.k.a. "cooked" mode (see termios(3)), where the
user's input only reaches the reader after a newline, and until then, the
erase character case be used to edit the line. This makes it easier for
non-perfect typist to use rhino ;-)

Various line-discipline features such as word-erase, line kill, flow
control and signals, aren't yet supported in this version.

Note how the echo and the line discipline (editing) are now both handled in
the console_poll() thread, while previously the echo was handled in the
read() implementation (console_read()). This means we now have full read-ahead,
i.e., the user can now type before read() starts - even during the OSv boot.

This patch also removes the old debugging read() function, which doesn't do
anything useful.