core: on each iteration processing /proc/self/mountinfo merge all discovery flags...

core: on each iteration processing /proc/self/mountinfo merge all discovery flags for each path

This extends on d253a45e1c147f5174265d71d7419da7bd52a88b, and instead of
merging just a single flag from previous mount entries of
/proc/self/mountinfo for the same path we merge all three.

This shouldn't change behaviour, but I think make things more readable.

Previously we'd set MOUNT_PROC_IS_MOUNTED unconditionally, we still do.

Previously we'd inherit MOUNT_PROC_JUST_MOUNTED from a previous entry on
the same line, we still do.

MOUNT_PROC_JUST_CHANGED should generally stay set too. Why that? If we
have two mount entries on the same mount point we'd first process one
and then the other, and the almost certainly different mount parameters
of the two would mean we'd set MOUNT_PROC_JUST_CHANGED for the second.
And with this we'll definitely do that still.

This also adds a comment explaining the situation a bit, and why we get
into this situation.