Don't set erc-server-announced-name unless known

Don't set erc-server-announced-name unless known

* lisp/erc/erc.el (erc-open): whenever this function is called, the
variable `erc-server-announced-name' may be set locally in the calling
server buffer.  However, if that buffer's dialed server matches that
of the one being created, the announced name is copied over on faith.
But there's no guarantee that the name will match the one ultimately
emitted by the server during its introductory burst.  Beyond
potentially causing confusion in protocol logs, this behavior may
complicate debugging efforts.  Setting the variable to nil helps
ensure a consistent environment when preparing a buffer for all newly
dialed connections.  This commit also simplifies the setting of
`erc-server-connected', which is always nil when connecting and
vice-versa.