testing/3proxy: fix init script and use supervise-daemon instead of start-stop-daemon

There are a couple of issues with the start-stop-daemon call in the init script, that
prevent it from working with 3proxy 0.8.13:
- 3proxy does not understand the --pidfile and --user options that are passed to it
  by start-stop-daemon (are they meant for start-stop-daemon instead?)
- Since 3proxy does not write its own pidfile by default, start-stop-daemon would
  need the -m option, so it would create the .pid file; instead of doing that
  supervise-daemon is now used instead, which avoids PID files altogether
- Both start-stop-daemon and supervise-daemon get confused when passing a user name
  starting with a number via the --user option and interpret it as a numeric UID; to
  avoid this, the UID/GID of the "3proxy" user/group are now fetched with
  ``id -u``/``id -g`` and then the correct numeric UID and GID are passed
7 jobs for !11951 with 3proxy-fix-200901 in 4 minutes and 45 seconds
detached
Status Job ID Name Coverage
  Lint
passed #198321
x86_64 docker-alpine
lint

00:00:23

 
  Build
passed #198326
aarch64 ci-build docker-alpine
build-aarch64

00:00:45

passed #198327
armv7 ci-build docker-alpine
build-armv7

00:02:16

passed #198325
ppc64le ci-build docker-alpine
build-ppc64le

00:00:59

passed #198324
s390x ci-build docker-alpine
build-s390x

00:03:39

passed #198323
x86 ci-build docker-alpine
build-x86

00:00:30

passed #198322
x86_64 ci-build docker-alpine
build-x86_64

00:00:45