zsh: add additional tmux escape hatch

Prevent auto-connecting to tmux if NOTMUX is set in the
environment. This allows .bdwconfig to set NOTMUX with variable
control and thus prevent .zlogin from running tmux.

The use case for this is the CSW buildfarm where using .notmux would
prevent tmux from being launced on the login host. Not using .notmux
would see each build host try to run a nested tmux. By conditionally
setting NOTMUX in .bdwconfig, the build hosts can avoid tmux while the
login host runs it normally.

Signed-off-by: Ben Walton <bdwalton@gmail.com>