Fails to run without `en_US@UTF-8` locale

[sumpfralle]

Since b4e30b4 tmux does not start anymore, if the locale en_US@UFT-8 is not locally available:

tmux: need UTF-8 locale (LC_CTYPE) but have ANSI_X3.4-1968

It is common for non-English setups to lack the above locale. Thus it would be nice to either present a better error message in such a case or (much better) use a more suitable locale (e.g. C.UTF-8).

Thank you for your time!