security/amavisd-new: service jails readiness + fixes

Rename rc script to comply to the rc scripting recommendations (netchild)
Make script service jail aware (netchild)
Remove amavis-milter.in that should most likely have been removed with
4c8100486c3b ~18 years ago, the script hasn't been part of the package
for a long time (flo)

PR: 280139