fsmgr: support multiple fs-types/mountpoint

Previous attempt was broken.
It would incorrectly be affected by mount failures.

This changes allows an fstab to contain multiple lines for a given
mount point.
The lines sharing a mount MUST be after each other.

The 1st matching line is the primary when it comes to mounting
and look ups for wiping.

Mounting based on a mount_point will attempt each dup in turn
until one succeeds.
The reported error will be that of the last failed attempt.

This is to allow quick experimentation between different FSes.

Bug: 15702546
Change-Id: I378d68ad13eb0098ec1ccb8dcf108b82acbe9ebb
Signed-off-by: JP Abgrall <jpa@google.com>