5404595b55eeefae69f2952a34dfc70e32523060 relocated code but kept
one backslah too many, leading to
$ tmux
error creating /run/user/$(id -u)/tmux-1000 (No such file or directory)
/run/user/$UID/ is created by pam_systemd(3) which also populates
XD_RUNTIME_DIR with that value.
Alternatively, TMUX_TMPDIR might simply default to XDG_RUNTIME_DIR
without providing the same directory yet again as default string in
parameter substitution, however such behaviour change is subject to
another patch.
In fact, with `security.polkit.enable = false` systemd_logind(8) fails
to start and /run/user/$UID/ is never created for unprivileged users
in proper login sessions; XDG_RUNTIME_DIR would consequently not be
set either.
Removing the fallback to /run/user/$UID/ would have caused TMUX_TMPDIR
to be empty, which in turn would lead tmux(1) to use /tmp/. This
effectively breaks the idea of isolated sockets entirely while hiding
errors from the user.
This basic module allows you to specify the tmux configuration.
As great as tmux is, some of the defaults are pretty awful, so having a
way to specify the config really helps.
