Refactor erc-select-read-args

Refactor erc-select-read-args

* lisp/erc/erc-backend.el (erc--server-connect-dumb-ipv6-regexp): Add
liberal pattern for matching bracketed IPv6 addresses.
(erc-server-connect): Remove brackets from IPv6 hosts before
connecting.
* lisp/erc/erc.el (erc--ensure-url): Add compat adapter to massage
partial URLs given as input that may be missing the scheme:// portion.
(erc-select-read-args): Keep bracketed IPv6 hosts
intact.  Make this function fully URL-aware (was only partially so).
Accept optional `input' argument.
* lisp/erc/erc-tests.el (erc-tests--ipv6-examples,
erc--server-connect-dumb-ipv6-regexp, erc-select-read-args): Add test
reading user input during interactive invocations of entry points.
(Bug#56514.)