Rename erc-channel-users to erc-channel-members

Rename erc-channel-users to erc-channel-members

* etc/ERC-NEWS: Mention name change.
* lisp/erc/erc-backend.el (erc-process-sentinel): Don't create an
empty `erc-channel-users' hash table in server buffers.  This is
arguably a bug fix as well as a minor breaking change.
* lisp/erc/erc-common.el (erc-get-channel-user,
erc-get-channel-member): Rename and alias former to latter.
* lisp/erc/erc.el (erc-channel-users, erc-channel-members): Rename
former to latter.  The old name invited much confusion because the
table's values are not mere `erc-channel-user' instances but cons
cells that include them and their corresponding server users.