Before this change the union's feature flags were a strict AND of the
underlying remotes. This means that a union of a local disk (which can
Move but not Copy) and a bucket based remote (which can Copy but not
Move) could neither Move nor Copy.
This fix advertises Move in the union if all the remotes can Move or
Copy. It also implements Move as Copy+Delete (like rclone does
normally) if the underlying union does not support Move.
This enables renames to work with unions of local disk and bucket
based remotes expected.
Fixes#5632
Before this change, when using an all create method with one of the
upstreams being read only, if there was an existing file on the read
only remote, it was impossible to update it.
This change detects that situation and creates the file on a
read/write upstream. This file will shadow the file on the read/only
upstream. If it is deleted the read only upstream file will be visible
again.
Fixes#4929
