First crack at `terminal-modes' in SSH. PuTTY now sends ERASE by default,

First crack at `terminal-modes' in SSH. PuTTY now sends ERASE by default,
Unix Plink sends everything sensible it can find, and it's fully configurable
from the GUI.

I'm not entirely sure about the precise set of modes that Unix Plink should
look at; informed tweaks are welcome.

Also the Mac bits are guesses (but trivial).

git-svn-id: svn://svn.tartarus.org/sgt/putty@5653 cda61777-01e9-0310-a592-d414129be87e