Moved the environment variables config block out of the Telnet panel

Moved the environment variables config block out of the Telnet panel
into the Connection panel, and implemented support for the SSH2
"env" request. (I haven't yet found a server which accepts this
request, so although I've visually checked the packet log and it
looks OK, I haven't yet been able to do a full end-to-end test.)
Also, the `pty' backend reads this data and does a series of
`putenv' commands before launching the shell or application.

This is mostly because in last week's UTF-8 faffings I got
thoroughly sick of typing `export LANG=en_GB.UTF-8' every time I
started a new testing pterm, and it suddenly occurred to me that
this would be precisely the sort of thing you'd want to have pterm
set up for you, particularly since you can configure it alongside
the translation settings and so you can ensure they match up
properly.

[originally from svn r4645]