mount: put all mounts which propagate from each other to a list

These information will help improving the restore of tricky mounts
configurations.

Function same_propagation_group checks if two mounts were created
simultaneousely through shared mount propagation, and the main part of
these - they should be in exaclty the same place inside the share of
their parents.

Function root_path_from_parent prints the mountpoint path
relative to the root of the parent's share, by first substracting
parent's mountpoint from our mountpoint and second prepending parents
root path (relative to the root of it's file system), e.g:

id	parent_id	root	mountpoint
1	0		/	/
2	1		/	/parent_a
3	1		/dir	/parent_b
4	2		/	/parent_a/dir/a
5	3		/	/parent_b/a

(Let 2 and 3 be a shared group)

For mount 4 root_path_from_parent gives:
"/parent_a/dir/a" - "/parent_a" == "/dir/a"
"/" + "/dir/a" == "/dir/a"

For mount 5:
"/parent_b/a" - "/parent_b" == "/a"
"/dir" + "/a" == "/dir/a"

So mounts 4 and 5 are a propagation group.

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