use sudo only when necessary

try sudo only when it seems necessary.  i.e. when we actually got EACCES.

motivations:

- the use of sudo involves some side effects which might be undesirable.
  for example, as sudo is a setuid binary, LD_PRELOAD is unset.
  it makes difficult to use this with a rump kernel.
  http://netbsd.gw.com/cgi-bin/man-cgi?rumphijack++NetBSD-current

- as priv/procket command can be used to open arbitrary /dev files,
  it's better to just allow a user to open eg. /dev/bpf (eg. by chmod)
  instead of allowing him sudo procket.