ensure that the .config path exists before writing

mkdir -p is safe in that it creates all intervening directories and does
not complain if they already exist.

Currently the write to ~/.config/systemd/user/tmux.service fails if the
~/.config/systemd/user directory isn't already existing.

scripts/handle_tmux_automatic_start/systemd_enable.sh

@@ -56,6 +56,8 @@ systemd_unit_file() {
 }
 
 write_unit_file() {
+	# it never hurts to make sure the directory we are writing into exists
+	mkdir -p "${systemd_config_path}"
   systemd_unit_file > "${systemd_unit_file_path}"
 }