privoxy: correctly format /etc/services additions

privoxy: correctly format /etc/services additions

This adds the missing protocol (e.g. /tcp and /udp) to the entry in
/etc/services. If the entry already exists, it will add the /tcp to it.
Otherwise, it will look and add the tcp and udp entries if either is
missing.

fixes: openwrt#19665

Signed-off-by: Josh Powers <powersj@fastmail.com>
(cherry picked from commit 95edfa723c9e7b1b99361a86022b9b20ac6f8ead)