readall and run hang when command doesn't exist

[mlubin]

julia> readall(`foobar`)

and

julia> run(`foobar`)

hang when there is no foobar executable in the path.

This should give an error instead.

[JeffBezanson]

I started looking into this. The problem seems to be that the child process inherits the parent's atexit handlers, and they only go away with a successful exec(). Since exec fails, when the child exits it runs pthread_cond_destroy inside openblas, which is probably trying to acquire a lock it can't get.

[JeffBezanson]

Seems to be fixed by calling _exit instead of exit, to avoid running cleanup.

[StefanKarpinski]

Thanks, Jeff.