Initial checkin of a native Mac OS X port, sharing most of its code

Initial checkin of a native Mac OS X port, sharing most of its code
with the Unix port and layering a Cocoa GUI on top. The basics all
work: there's a configuration panel and a terminal window, the
timing interface works and the select interface functions. The same
application can run both SSH (or other network) connections and
local pty sessions, and multiple sessions in the same process are
fully supported.

However, it's horribly unfinished in a wide variety of other ways;
anyone interested is invited to read README.OSX and wince at the
length and content of its `unfinished' list.

[originally from svn r5308]