FR: make exit-node list hostname, unique node identifiers and not -ts.net addresses

### What are you trying to do?

In the output of exit-node list, the exist nodes are listed with their -ts.net names under the hostname column. At the bottom of the output says to use an exit node, use the set command followed by the hostname or IP. That can be misleading and lead to users trying to set the exit node with the -ts.net name from the exit-node list command. Instead hostname should show the host name without the -ts.net suffix.

### How should we solve this?

In cmd/tailscale/cli/exitnode.go - instead of using peer.DNSName, use peer.HostName.

### What is the impact of not solving this?

Attempting to set an exit node, failing, then trying out the host name without the magicdns suffix again. This might be annoying to users.

### Anything else?

_No response_

Open cmd/tailscale/cli/exitnode.go and trace the exit-node list output; the issue identifies peer.DNSName as the current source and peer.HostName as the desired source. Run the exit-node list command and verify that the hostname column omits the -ts.net suffix and shows the name users should pass to set.

Written by the indexing model from the issue text.