ipn,tailfs: tie TailFS share configuration to user profile

Previously, the configuration of which folders to share persisted across
profile changes. Now, it is tied to the user's profile.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -829,6 +829,10 @@ func TestPrefFlagMapping(t *testing.T) {
 			// Handled by TS_DEBUG_FIREWALL_MODE env var, we don't want to have
 			// a CLI flag for this. The Pref is used by c2n.
 			continue
+		case "TailFSShares":
+			// Handled by the tailscale share subcommand, we don't want a CLI
+			// flag for this.
+			continue
 		}
 		t.Errorf("unexpected new ipn.Pref field %q is not handled by up.go (see addPrefFlagMapping and checkForAccidentalSettingReverts)", prefName)
 	}