Accommodate nonstandard turbo file senders in erc-dcc

Accommodate nonstandard turbo file senders in erc-dcc

* lisp/erc/erc-dcc.el (erc-dcc-list): Document optional :turbo item.
(erc-message-english-dcc-list-{head,line,item}): Adjust format strings
to make room for "(T)" turbo indicator.
(erc-dcc-do-GET-command): Optionally set :turbo in `erc-dcc-list'
entry when passed "-t" in the "/DCC GET" slash command.  Also add
switch to command line in front-matter Commentary, but refrain from
publicizing further because our implementation is only defensive and
only for receiving.
(erc-dcc-do-LIST): Print message with new format specifier for turbo
status.
(erc-dcc-ctcp-query-send-regexp): Account for T- and S-prefixed
commands.  Receiving from an SSEND-capable sender will be added in a
subsequent commit.
(erc-dcc-handle-ctcp-send): Set :turbo item in `erc-dcc-list' member
when new match group is nonempty.
(erc-dcc--X-send-final-turbo-ack): New internal variable and potential
future option for extreme corner cases involving maverick turbo
senders, like WeeChat, who don't use the TSEND command variant.
(erc-dcc-get-filter): Don't send when turbo is active.

* test/lisp/erc/erc-dcc-tests.el: Add new file.
(Bug#54458)