udisks2: URI unescape mount options read from /etc/fstab

For example, if the /etc/fstab file contains this line

 defaults,users,comment=gvfs.name=Photos%2C%20Media%20and%20Stuff,comment=gvfs.icon_name=folder-videos

then this call

 lookup_mount_option_value(mount_point, "comment=gvfs.name=")

will return the string

 'Photos, Media and Stuff'

as expected.

See https://bugzilla.gnome.org/show_bug.cgi?id=660536#c11

 for details.

Signed-off-by: David Zeuthen <davidz@redhat.com>