[Fstab] Show mounted "overlay" filesystems

Summary:
Overlay filesystems are not enumerated by udisks, so add support for these
to the fstab backend

See also D15739

Test Plan:
$> grep overlay /proc/self/mounts

NAME /mnt overlay rw,relatime,lowerdir=/home/stefan/tmp/,upperdir=/tmp/overlay/rw,workdir=/tmp/overlay/work 0 0
NAME /home/stefan/tmp2 overlay rw,relatime,lowerdir=/home/stefan/tmp/,upperdir=/tmp/overlay2/rw,workdir=/tmp/overlay2/work 0 0

$> solid-hardware5 list details
...
udi = '/org/kde/fstab/overlay/mnt'

parent = '/org/kde/fstab'  (string)
vendor = 'overlay'  (string)
product = '/mnt'  (string)
description = '/mnt (overlay)'  (string)
icon = 'folder'  (string)
StorageAccess.accessible = true  (bool)
StorageAccess.filePath = '/mnt'  (string)
StorageAccess.ignored = false  (bool)

udi = '/org/kde/fstab/overlay/home/stefan/tmp2'

parent = '/org/kde/fstab'  (string)
vendor = 'overlay'  (string)
product = '~/tmp2'  (string)
description = '~/tmp2 (overlay)'  (string)
icon = 'folder'  (string)
StorageAccess.accessible = true  (bool)
StorageAccess.filePath = '/home/stefan/tmp2'  (string)
StorageAccess.ignored = false  (bool)

Reviewers: ngraham, fvogt, broulik

Reviewed By: ngraham, fvogt, broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22080