dpkg-shlibdeps uses softer warnings for libraries that are likely plugins

dpkg-shlibdeps uses softer warnings for libraries that are likely plugins

* scripts/dpkg-shlibdeps.pl: If the binary analyzed is a library on a
non-public path, assume it's likely a plugin and reduce the number of
warnings displayed about symbols not found. Also use a different,
hopefully less confusing message.
* man/dpkg-shlibdeps.1: Document the above changes.