ipn/ipnstate: add PeerStatus.AltSharerUserID, stop mangling Node.User

In b987b2ab18 (2021-01-12) when we introduced sharing we mapped
the sharer to the userid at a low layer, mostly to fix the display of
"tailscale status" and the client UIs, but also some tests.

The commit earlier today, 7dec09d169, removed the 2.5yo option
to let clients disable that automatic mapping, as clearly we were never
getting around to it.

This plumbs the Sharer UserID all the way to ipnstatus so the CLI
itself can choose to print out the Sharer's identity over the node's
original owner.

Then we stop mangling Node.User and let clients decide how they want
to render things.

To ease the migration for the Windows GUI (which currently operates on
tailcfg.Node via the NetMap from WatchIPNBus, instead of PeerStatus),
a new method Node.SharerOrUser is added to do the mapping of
Sharer-else-User.

Updates #1909
Updates tailscale/corp#1183

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -21,6 +21,7 @@ import (
 	"tailscale.com/types/opt"
 	"tailscale.com/types/structs"
 	"tailscale.com/types/tkatype"
+	"tailscale.com/util/cmpx"
 	"tailscale.com/util/dnsname"
 )
 
@@ -223,9 +224,9 @@ type Node struct {
 	// e.g. "host.tail-scale.ts.net."
 	Name string
 
-	// User is the user who created the node. If ACL tags are in
-	// use for the node then it doesn't reflect the ACL identity
-	// that the node is running as.
+	// User is the user who created the node. If ACL tags are in use for the
+	// node then it doesn't reflect the ACL identity that the node is running
+	// as.
 	User UserID
 
 	// Sharer, if non-zero, is the user who shared this node, if different than User.
@@ -386,12 +387,20 @@ func (n *Node) IsTagged() bool {
 	return len(n.Tags) > 0
 }
 
+// SharerOrUser Sharer if set, else User.
+func (n *Node) SharerOrUser() UserID {
+	return cmpx.Or(n.Sharer, n.User)
+}
+
 // IsTagged reports whether the node has any tags.
 func (n NodeView) IsTagged() bool { return n.ж.IsTagged() }
 
 // DisplayName wraps Node.DisplayName.
 func (n NodeView) DisplayName(forOwner bool) string { return n.ж.DisplayName(forOwner) }
 
+// SharerOrUser wraps Node.SharerOrUser.
+func (n NodeView) SharerOrUser() UserID { return n.ж.SharerOrUser() }
+
 // InitDisplayNames computes and populates n's display name
 // fields: n.ComputedName, n.computedHostIfDifferent, and
 // n.ComputedNameWithHost.