Fix all shellcheck warnings, add TMUX_CMD variable to set the binary for tmux (like ssh)

[fernsehmuell]

Moin hukl,

this PR fixes all shellcheck warnings and also adds a variable to set the tmux binary (as you already had for ssh). The TMUX_CMD makes the nix package a little simpler. I can set the path for tmux at one place and don't have to add it in so many places.