ipn, cmd/tailscale/cli: allow setting FQDN sans dot as an exit node

In #10057, @seigel pointed out an inconsistency in the help text for
`exit-node list` and `set --exit-node`:

1.  Use `tailscale exit-node list`, which has a column titled "hostname"
    and tells you that you can use a hostname with `set --exit-node`:

    ```console
    $ tailscale exit-node list
     IP                  HOSTNAME                               COUNTRY            CITY                   STATUS
     100.98.193.6        linode-vps.tailfa84dd.ts.net           -                  -                      -
    […]
     100.93.242.75       ua-iev-wg-001.mullvad.ts.net           Ukraine            Kyiv                   -

    # To view the complete list of exit nodes for a country, use `tailscale exit-node list --filter=` followed by the country name.
    # To use an exit node, use `tailscale set --exit-node=` followed by the hostname or IP.
    # To have Tailscale suggest an exit node, use `tailscale exit-node suggest`.
    ```

    (This is the same format hostnames are presented in the admin
    console.)

2.  Try copy/pasting a hostname into `set --exit-node`:

    ```console
    $ tailscale set --exit-node=linode-vps.tailfa84dd.ts.net
    invalid value "linode-vps.tailfa84dd.ts.net" for --exit-node; must be IP or unique node name
    ```

3.  Note that the command allows some hostnames, if they're from nodes
    in a different tailnet:

    ```console
    $ tailscale set --exit-node= ua-iev-wg-001.mullvad.ts.net
    $ echo $?
    0
    ```

This patch addresses the inconsistency in two ways:

1.  Allow using `tailscale set --exit-node=` with an FQDN that's missing
    the trailing dot, matching the formatting used in `exit-node list`
    and the admin console.

2.  Make the description of valid exit nodes consistent across commands
    ("hostname or IP").

Updates #10057

Change-Id: If5d74f950cc1a9cc4b0ebc0c2f2d70689ffe4d73
Signed-off-by: Alex Chan <alexc@tailscale.com>

ipn/prefs_test.go

@@ -1009,7 +1009,7 @@ func TestExitNodeIPOfArg(t *testing.T) {
 			name:    "no_match",
 			arg:     "unknown",
 			st:      &ipnstate.Status{MagicDNSSuffix: ".foo"},
-			wantErr: `invalid value "unknown" for --exit-node; must be IP or unique node name`,
+			wantErr: `invalid value "unknown" for --exit-node; must be IP or hostname`,
 		},
 		{
 			name: "name",
@@ -1041,6 +1041,21 @@ func TestExitNodeIPOfArg(t *testing.T) {
 			},
 			want: mustIP("1.0.0.2"),
 		},
+		{
+			name: "name_fqdn_sans_dot",
+			arg:  "skippy.foo",
+			st: &ipnstate.Status{
+				MagicDNSSuffix: ".foo",
+				Peer: map[key.NodePublic]*ipnstate.PeerStatus{
+					key.NewNode().Public(): {
+						DNSName:        "skippy.foo.",
+						TailscaleIPs:   []netip.Addr{mustIP("1.0.0.2")},
+						ExitNodeOption: true,
+					},
+				},
+			},
+			want: mustIP("1.0.0.2"),
+		},
 		{
 			name: "name_not_exit",
 			arg:  "skippy",
@@ -1067,7 +1082,7 @@ func TestExitNodeIPOfArg(t *testing.T) {
 					},
 				},
 			},
-			wantErr: `invalid value "skippy.bar." for --exit-node; must be IP or unique node name`,
+			wantErr: `invalid value "skippy.bar." for --exit-node; must be IP or hostname`,
 		},
 		{
 			name: "ambiguous",