lockfile_unix: add a couple of additional safety checks
Lock the mounts list with its own lockfile
Update a comment
layers/images: don't try to clean up with just a read-only lock
Switch to read-only locks in some places
Locker.Locked(): clarify that we're checking for write locks
CLI helper: print backtraces with errors
abs.bats: don't forget to shut things down after the test
stores.bats: hard fail if we unexpected can't use read-only stores
lockfile: use a sync.RWMutex
GetStore(): resolve passed-in paths to absolute paths
