Add a new facility vm_object_set_path() that can be used to set an
informational path to be displayed by procstat -v. Allow the path to
be an arbitrary string, or a jail-adjusted path. Use it to show
shm_open() paths.
This is a WIP. I am not yet sure about the sanity of the memory allocated to hold the copy of the path or the locking. I considered trying to share the same path string for the hash table and for the vmobject, but that seemed messy, hence new copy. Clues and better ideas welcome! I wonder if it would be desirable to show the path differently if it has been unlinked from the hash table. I am not sure if it would ever be desirable to show paths for any other kind of vmobject, and that's why I made it an optional flag to control whether it needs to be adjusted for jails (because some other hypothetical user of this facility might not use uipc_shm.c's fake jail root dir concept).