FYI: new openat-like function: mkdirat

[Roland McGrath]

> So I guess the exec*at business would ultimately be more complicated,
> with two file descriptor parameters: one identifying the working
> directory, and another by which to interpret the first parameter
> if it's a relative file name.

It seems adequate to just use chdir/fchdir for changing cwd, and then
execveat given the file name (you can get an fd for the original cwd before
chdir, for relative paths in exec).  

But note that we already have fexecve.