attach-descriptor should attach only for pom packaging by default (and provide a way to force eventually)

in Maven 2, attach-decriptor goal is bound to default lifecycle for pom only
with Maven 3, we have to explicitely add the goal to the build section, but by default, it will be inherited to every sub-module (jar, ...) and not only pom, which is not ideal: it is useful only for pom packaging in general
without this restriction, we need to configure the goal in pom with inherited=false, then re-configure in every pom sub-module: what a pain

if we want to permit site descriptor attachement even for non-poms (I don't know if this is useful, AFAIK it isn't but...), we can add a pomOnly parameter defaulting to true