Revert "all: keep UserProfiles a slice instead of a map for longer"

This reverts commit e5894aba42.

Breaks macOS/iOS build. Reverting per chat with Josh; he'll fix later today.

control/controlclient/netmap.go

@@ -44,9 +44,11 @@ type NetworkMap struct {
 
 	// ACLs
 
-	User         tailcfg.UserID
-	Domain       string
-	UserProfiles []tailcfg.UserProfile
+	User   tailcfg.UserID
+	Domain string
+	// TODO(crawshaw): reduce UserProfiles to []tailcfg.UserProfile?
+	// There are lots of ways to slice this data, leave it up to users.
+	UserProfiles map[tailcfg.UserID]tailcfg.UserProfile
 	// TODO(crawshaw): Groups       []tailcfg.Group
 	// TODO(crawshaw): Capabilities []tailcfg.Capability
 }