[WIP] Resolve mount conflicts in mount manager

:warning: EXPERIMENTAL :warning: 

Experimental fix to resolve mount point conflicts in the mount manager.
@icewind1991 @schiesbn can you give me some feedback about this ?
Do you see any case where this could go wrong, apart from race conditions ?

Commit comment:

Whenever a mount point is added, call file_exists() to check for an
existing file/folder/mount point.

In case of conflict, try and rename to newly added mount point. Else
rename the target.

TODO:
- [ ] add unit tests (once the case is accepted)
- [ ] extensive regression testing of all combinations of mount type conflicts
- [ ] extensive regression testing of mount types conflict with folder/filer
Where is `$user` used?