Infrastructure and commands to manage the environment for processes started
within tmux.
There is a global environment, copied from the external environment when the
server is started and each session has an (initially empty) session
environment which overrides it.
New commands set-environment and show-environment manipulate or display the
environments.
A new session option, update-environment, is a space-separated list of
variables which are updated from the external environment into the session
environment every time a new session is created - the default is DISPLAY.
Add a terminal-overrides session option allowing individual terminfo(5) entries
to be overridden. The 88col/256col checks are now moved into the default
setting and out of the code.
Also remove a couple of old workarounds for xterm and rxvt which are no longer
necessary (tmux can emulate them if missing).
