- introduce API for "making room", that grows/shrinks home directory
according to elastic parameters, discards blocks, and removes additional snapshots. Call it
either from UI when disk space gets low
+ - when homed is in use, maybe start the user session manager in a mount namespace with MS_SLAVE,
+ so that mounts propagate down but not up - eg, user A setting up a backup volume
+ doesn't mean user B sees it
* homed: during login resize fs automatically towards size goal. Specifically,
resize to diskSize if possible, but leave a certain amount (configured by a