TmuxRunning: handle multi-word commands

Just as in f8ed7446755eeb88191e16749350efa1e7e6197c, tmux wants a single
argument for its command. This applies to the "split-window" command as
well as "new."

Note that this alone is not enough to fix the TmuxRunning devshell when
using pseudo because tmux does not preserve the environment that pseudo
requires.

Signed-off-by: Tyler Hall <tylerwhall@gmail.com>
terminal: Run command using a wrapper script

Some terminals may not pass the environment into the child process. This
is true when using "tmux split-window." If tmux is already running, it
will start the command with the tmux session environment, ignoring the
environment where the command was issued.

This could possibly be worked around when launching tmux by injecting
variables into the user's session environment or adding the variables to
the "update-environment" tmux setting. However, both methods would
permanently alter the user's session, which is undesirable.

By using a wrapper script, we have full control over the final
environment. Replace the env dictionary with an empty data smart that
will contain the exported variables and a wrapper function that execs
the original command.

Signed-off-by: Tyler Hall <tylerwhall@gmail.com>