Drop privileges even earlier.

Do it at the same point at which Wireshark does so.  Do some other
things in the same order as well.

Change-Id: I2925366d49d14271ceffa1a938b5e3450337c772
Reviewed-on: https://code.wireshark.org/review/2743
Reviewed-by: Guy Harris <guy@alum.mit.edu>