Since elvish does not support job control it should run all external
commands with the tty job control signals set to be ignored. See "APUE",
third edition, page 379.
This doesn't actually fix the problem in the case of broken programs
like vim which blindly assume the parent process implements job control.
But there isn't much we can do about that other than try to get those
programs to pay attention to being told to ignore SIGTSTP.
Fixes#988
