mount: improve can_mount_now using propagation groups

1) redo waiting for parents of propagation group to be mounted using
pre-found propagation groups
2) for shared mount wait for children of that shared group which has no
propagation in our shared mount

(2) - effectively is a support of non-uniform shares, that means two
mounts of shared group can have different sets of children now - we will
mount them in the right order, but propagate_mount and validate_shared
are still preventing c/r-ing such shares, will fix the former and remove
the latter in separate(next) patches.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>