The DN of a search result entry, or None if the entry is malformed. SearchEntry::construct
panics on a malformed entry instead, which would end the process.
Refuses anything beyond a host name, IPv6 literal and :port rather than normalising it, so a
typo can’t downgrade the connection or send the bind elsewhere.
AD answers every subtree search with continuation references to subordinate naming contexts,
so a domain-root base would look ambiguous. References are dropped rather than chased, so the
bind never goes to a server nobody named.