Fix command-line parsing regression in erc-cmd-DCC

Fix command-line parsing regression in erc-cmd-DCC

* lisp/erc/erc-compat.el (erc-compat--28-split-string-shell-command,
erc-compat--split-string-shell-command): Remove unused function and
macro.
* lisp/erc/erc-dcc.el (erc-cmd-DCC): Use own arg-parsing function.
* lisp/erc/erc.el (erc--shell-parse-regexp,
erc--split-string-shell-cmd): New regexp constant and arg-parsing
function based on those in shell.el.
* test/lisp/erc/erc-dcc-tests.el
(erc-dcc-tests--erc-dcc-do-GET-command): Accept new `nuh' argument
representing message source/sender.
(erc-dcc-do-GET-command): Add tests for regression involving pipe
character.
* test/lisp/erc/erc-tests.el (erc--split-string-shell-cmd): New test.
(Bug#62444)

Thanks to Fernando de Morais for reporting this bug.