tlsenum package probes a remote endpoint with one ClientHello
per (version, cipher) pair via utls, so the checker can report the
exact set the server accepts rather than only the suite Go's stdlib
happens to negotiate. Probe accepts an Upgrader callback so STARTTLS
dialects plug in without tlsenum learning about them; the checker
bridges its existing dialect registry through upgraderFor.
Bound line reads with readLineLimited to prevent a peer from exhausting
memory by withholding line terminators, wrap previously bare error
returns for consistent context, surface XML decoder Skip errors, and
replace the goto in the XMPP feature scan with a labeled break. New
starttls_test.go exercises SMTP/IMAP/POP3/XMPP/LDAP success and
not-advertised paths through net.Pipe-mocked servers.
