racct: Add new "persistent" property for resources

[cyril_freebsdfoundation.org]

Summary

This adds a new "persistent" property that describes resources which can drop in usage but do not automatically get reclaimed when a process dies. They must instead be manually dropped. This correctly handles shared resources such as shared memory. Previously, shared memory resources would be automatically dropped when a process died and then dropped again whenever the shared memory object was closed, leading to incorrect accounting.

[trasz]

What's the difference between this and RESOURCE_SLOPPY?

[cyril_freebsdfoundation.org]

In D30777#693823, @trasz wrote:

What's the difference between this and RESOURCE_SLOPPY?

The main difference is that RACCT_IS_SLOPPY is not in RACCT_CAN_DROP. An alternate solution could be to add RACCT_SLOPPY to RACCT_CAN_DROP and remove RACCT_RECLAIMABLE from RACCT_NSHM and RACCT_SHMSIZE.

[trasz]

I wonder, should it perhaps also apply to some other SysV IPC resources?