Make erc-update-channel-current-member stricter

Make erc-update-channel-current-member stricter

* lisp/erc/erc.el (erc--update-cusr-status-if-changed): Remove unused
macro.
(erc-update-current-channel-member): Redo doc string and abide by its
original language to the letter by not honoring a value of t for the
five channel-membership status parameters, even though this may break
user code that accidentally passes t instead of `on'.  Avoid double
lookup for nick in `erc-server-users'.  Rename some variables as per
recommended conventions for `erc-channel-user' objects.  Stash
downcased nick for further reuse.  Don't bother factoring in `addp' to
return value because `cusr-changed-p' is always non-nil when `addp'
is.

(cherry picked from commit 6c899525571749c29ec3e2826c4cc70b85b17db1)