look(1): use a stricter pledge(2) in lieu of unveil(2)

look(1): use a stricter pledge(2) in lieu of unveil(2)

We're only reading one file here, so unveil(2) is overkill.  We can
achieve the same effect with just pledge(2):

- Start with an initial pledge(2) of "stdio rpath" at the top of main().
  We know we need to read a file at this point but don't yet know which
  one.

- Drop the pledge(2) down to "stdio" after we open(2) and fstat(2) the
  chosen file.

- Dropping "rpath" obviates unveil(2).

Thread: https://marc.info/?l=openbsd-tech&m=164437072017248&w=2

ok millert@