Return the command client return code with MSG_EXIT now that MSG_ERROR and
MSG_PRINT are unused.
New clients should be compatible with old tmux servers but vice versa may print
an error.
Add a flag for jobs that shouldn't be freed after they've died and use it for
status jobs, then only kill those jobs when status-left, status-right or
set-titles-string is changed.
Fixes problems with changing options from inside #().
