Rewrite a confusing loop when freeing the arg array on exit and move the check
for argv being NULL, prompted by parfait via deraadt.
Also fix some definite brokenness when assigning multiple environment variables
in arguments (such as "X=1 Y=2").
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).
