Allow login to contain NUH delimiters in erc-parse-user

Allow login to contain NUH delimiters in erc-parse-user

* lisp/erc/erc.el (erc--parse-user-regexp-legacy,
erc--parse-user-regexp-pedantic): New constants.  The first is the
original pattern that matches across line endings.  The second
disallows multiline strings and interprets excess delimiting
characters as part of the middle, "user" component as per RFC1459.
The latter distinction is largely academic because most servers reject
such logins anyway, but bridges to other protocols and future
extensions may need to exploit this for novel uses.
(erc--parse-user-regexp): New variable, currently set to
`erc--parse-user-regexp-legacy'.
(erc-parse-user): Keep original pattern as default, but do so
indirectly via `erc--parse-user-regexp'.
* test/lisp/erc/erc-tests.el (erc-parse-user): New test.