Permit personal mount points to be used for sharing

An issue existed where `readData` used `OCP\User::getUser()` to get the user
for personal mount points, which worked in all situations apart from when a
personal mount point was used for sharing, so the return from `getUser()` is
not the user that owns the share. As such, any personal mount points would not
work correctly when shared.

`readData` and `writeData` have been changed from using a `$isPersonal`
boolean to using a `$user` string|null. `$isPersonal = false` can now be
written as `$user = NULL` (or left out in the case of `readData`), and
`$isPersonal = true` can be written as `$user = OCP\User::getUser()`.

Backport of abfd7ec from master
Merge pull request #9635 from owncloud/stable6-files_external_fix_readData

[stable6 backport] Permit personal mount points to be used for sharing