Fix DCC GET flag parsing in erc-dcc

Fix DCC GET flag parsing in erc-dcc

* lisp/erc/erc-dcc.el (erc-cmd-DCC): Tokenize raw input line but also
accommodate legacy invocation.
(pcomplete/erc-mode/DCC): Quote file names when suggesting.  Account
for double-hyphen "end-of-options"-like separator.
(erc-dcc-do-GET-command): Simplify signature, subsuming NICK in
variadic args, now ARGS instead of FILE, which changes the arity
from (2 . many) to (1 . many).  Explain usage in doc string.  Honor an
optional separator, "--", if present.  (Bug#62444.)
* test/lisp/erc/erc-dcc-tests.el (erc-dcc-do-GET-command): Call new
parameterized helper with various flag/file combinations.
(erc-dcc-tests--erc-dcc-do-GET-command): New fixture function.
(pcomplete/erc-mode/DCC--get-quoted,
pcomplete/erc-mode/DCC--get-sep): New tests.