TmuxRunning: handle multi-word commands

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>
Signed-off-by: Saul Wold <sgw@linux.intel.com>