partdisks: use path instead of mapper_path on LUKS devices to do checking and locking

This may be slower, but now linstaller can lock and properly check mountpoints on
an encrypted partition unlocked by foreign applications.
partdisks: warn the user if LUKSdrive.close() failed.
bricks: inst: ensure we remove every dependencies of the features' meta-packages.
userhost: inst: set groups if root is disabled.
4.99.6 (RC7)
Merge branch 'devel' into debian-devel
4.99.6-1