Move the "resolve a connectat(2) target to a referenced peer socket"
half of unp_connectat() -- the namei() lookup and unp_vnode_peer()
call -- into a helper, leaving unp_connectat() with the connection state
machine plus a single unp_connect_peer().

This is where the next change grows the ways a peer can be named; keeping
it a helper up front keeps that change focused on the new resolution
logic.

No functional change intended.

Signed-off-by: John Ericson <John.Ericson@Obsidian.Systems>
Assisted-by: Claude Code (Claude Opus 4.8 and Fable 5)