Allow the TERM env variable to be overwritten by .ssh/config

[Palmer Dabbelt]

This is pretty much the same request as an old mailing list thread

http://marc.info/?l=openssh-unix-dev&m=134073696320230&w=2

for almost exactly the same reason: some machines don't have
new-enough terminal info databases to support newer TERM strings
("rxvt-unicode-256color" in my case).  As such, I'd like to be able to
override the TERM that is forwarded to some machines from my
.ssh/config.

The solutions proposed in that post don't work well for me because I
like having a shared .bashrc and don't want to go making it super long
just to have a big list of hosts that don't support "*-256color".

Following is a git-formatted patch to implement this feature.  It's
based against the 6.6-p1 tarballs as I'm not really sure what else to
base it against.  Hopefully that's OK?