Prune the volumes nobody named, and the rest only when asked

[MayCXC]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Closes #2169.

A volume mounted without a name is created because a container mounts it, and is of no use to anyone once nothing mounts it. A volume someone named is theirs, and one that nothing mounts yet is still theirs to mount later, so removing it because nothing mounts it takes away something they asked for and did not ask to lose.

Prune takes the anonymous ones, and the named ones only when asked for all of them. That is what pruning images does here already: dangling by default, and every unused image when asked for all of them. Volumes carry the label that tells them apart from the moment one is created without a name, so nothing new is recorded to do it.

Pruning containers and networks needs none of this. Every network is named when it is made and every stopped container was asked for by name, so neither has a kind that nobody asked for.

Marked breaking because container volume prune with no flags removes strictly less than it did: an unused named volume now survives until --all.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

The serial suite asks for all of them where it means an unused volume it named, and a new case covers the split itself: an anonymous volume goes, the named one beside it stays, and asking for all takes that one too.

Integration suite: 397 passed. Unit suite: 772 passed. make fmt, make check clean.