find the tmux path even when the shell is tcsh

[Gullumluvl]

The shell command used to detect the path to the tmux executable is available in Bash but not Tcsh: command -v. So I proposed to use whereis instead, which should be available in both shells. However it's an external program, not a builtin, so I am not sure of its availability under all systems (I have Ubuntu 16.04).