mark: separate mark:moving from mark:arrived

mark: separate mark:moving from mark:arrived

The mark:moving notification was originally for when a specific mark
moved at all.
For b64 I extended it to also notify when *any* mark moved to a specific
mark.
This can cause illegal nested notification if a mark:moving handler
tries to move a (non-specific) mark.  That is bad.

So separate the two: Add mark:arrived for when a mark is moved to a
flagged marks.

Also make sure all new marks aren't flagged.  I invert the sense of the
flag and call it "MARK_FLAG_WATCHED".  This will be clear by default,
and only set once the (renamed) mark_watch() is called.

Signed-off-by: NeilBrown <neil@brown.name>