allow infinite number of tasks by default in tmux@.service; rel 2

allow infinite number of tasks by default in tmux@.service; rel 2

by default units are assigned DefaultTasksMax limit of 4915 tasks (might
be different depending on other conditions see
https://www.freedesktop.org/software/systemd/man/systemd-system.conf.html#DefaultTasksMax=)
however user sessions allow infinite number of tasks. model
tmux@.service after user session by default since in a way it is a user
session.