Fix previous change to erc (where commands like /me wouldn't be sent)

Fix previous change to erc (where commands like /me wouldn't be sent)

* lisp/erc/erc-ring.el (erc-add-to-input-ring):
* lisp/erc/erc-goodies.el (erc-send-distinguish-noncommands): Pass
in a erc-input structure instead of a simple string.
* lisp/erc/erc.el (erc-pre-send-functions): Document the new
argument to the filter functions.
(erc-send-input): Use the new structure to allow the filter
functions to alter all three things: The string, whether to insert
the string, and whether to send the string.