bash-completion: (umount) use findmnt, escape a space in paths

 # mount /dev/sdc1 /mnt/test/foo\ bar
 # umount <tab>

has to return "/mnt/test/foo\ bar".

Changes:

 * don't use mount | awk output, we have findmnt
 * force compgen use \n as entries separator

Addresses: https://github.com/karelzak/util-linux/issues/539
Signed-off-by: Karel Zak <kzak@redhat.com>