bug#41960: nntp.el: Switch back to CAPABILITIES command?

[Kyle Meyer <kyle@kyleam.com>]

In Emacs 24.4 [0], the command used in nntp.el for detecting NNTP
capabilities changed from CAPABILITIES to HELP to work around an issue
with a particular news server not recognizing the CAPABILITIES command
(bug#12763).

This probably works most the time because, according the RFC 3977 [1],
it is "customary for [HELP] to list the valid commands and give a brief
definition of what they do".  However, that same section says HELP
shouldn't be used as a replacement for CAPABILITIES.

A specific case where using HELP instead of CAPABILITIES causes an issue
is with public-inbox's [2] NNTP service, such as the one hosted at
<news.public-inbox.org>.  public-inbox doesn't include a list of
commands in its HELP output.  That is, it's not following the
convention, but it is still following the specification.  Because
nntp-open-connection tries to detect capabilities with HELP, it doesn't
catch the properly advertised STARTTLS capability and fails to upgrade
to a TLS connection.

I'm guessing that the server from bug#12763 didn't recognize
CAPABILITIES because it was based on RFC 977.  However, given RFC 3977
supersedes 977 and is from 2006, does it make sense at this point to
assume CAPABILITIES is supported?


[0] b9989bfa7e59b423df76b184e657b66017f72ff9
    lisp/gnus/nntp.el (nntp-open-connection): Fix nntp connection
    problems with the Typhoon server
    2012-12-28
[1] https://tools.ietf.org/html/rfc3977#section-7.2
[2] https://public-inbox.org/