alpine packaging: use a more standard packaging format

alpine packaging: use a more standard packaging format

Currently, we just package the frr daemons, but we don't run
them.  This is fine for basic tests, but it is inconvenient to
orchestrate the daemons from downstream test environments.

Here, we follow the redhat and debianpkg formats more closely,
putting the daemons in /usr/lib/frr and including the frr user
and groups in the package.  We also include a docker specific
startup script and a sysvinit link in /etc/init.d/frr for
openrc based alpine installs.

Testing done:

Built packages, built base images, everything seems to work fine.
Uninstalled the package, all the daemons stopped.

Issue: https://github.com/FRRouting/frr/issues/2030
Signed-off-by: Arthur Jones <arthur.jones@riverbed.com>