Appending KRB5CCNAME to the list of environment variables to be
preserved in tmux sessions is more elegant than replacing the
original list.
set -s escape-time 100
# Ensure KRB5CCNAME is preserved in the environment.
-set -g update-environment 'DISPLAY WINDOWID SSH_ASKPASS SSH_AUTH_SOCK SSH_AGENT_PID SSH_CONNECTION KRB5CCNAME'
+set -a -g update-environment ' KRB5CCNAME'
# Clock.
setw -g clock-mode-colour white