Optionally prompt for more ERC entry-point params

Optionally prompt for more ERC entry-point params

* doc/misc/erc.texi: Update statement about availability of `:user'
keyword param when entry points called interactively.
* lisp/erc/erc/compat.el: Don't require `url-parse' when compiling.
Add forward declaration for `url-type'.
* lisp/erc/erc.el: Don't require `url-parse' when compiling.  Add
forward declarations for accessors of `url' struct from `url-parse'
library.
(erc-select-read-args): Allow optionally calling entry points with a
prefix arg to access params `user' and `:full-name'.
(erc-tls): Update doc string.
* test/lisp/erc/erc-tests.el (erc-select-read-args): Add test for
extra args.  (Bug#60428.)