Allow setting `erc-split-line-length' to zero

Allow setting `erc-split-line-length' to zero

* etc/ERC-NEWS: Mention that `erc-flood-protect' no longer affects
line splitting.
* lisp/erc/erc-backend.el (erc-split-line-length): Mention ways for
modules to suppress line splitting entirely.
(erc--split-line): Exit loop instead of asserting progress has been
made.
* lisp/erc/erc.el (erc--split-lines): Don't split input when
option `erc-split-line-length' is zero.
* test/lisp/erc/erc-tests.el (erc--split-line): Assert behavior when
`erc-split-line-length' is 0.  (Bug#62947)