Allow irc network symbols in erc-autojoin-channels-alist

Allow irc network symbols in erc-autojoin-channels-alist

* lisp/erc/erc-join.el (erc-autojoin-channels-alist): Explain the
extension.
(erc-autojoin-server-match): New function.
(erc-autojoin-channels): Use it.
(erc-autojoin-current-server): New function.
(erc-autojoin-add): Use it.
(erc-autojoin-remove): Ditto.

This can be useful when connecting to an IRC proxy like Weechat that
relays several networks under the same server. If we just keyed on the
server name, we would end up joining a channel on all networks
whenever we join one network on that server.

Networks are simply stored as symbols instead of regexes, since that's
how `erc-network' works.

The `erc-autojoin-add' function will still auto-add servers as strings
if the network doesn't have at least one entry in
`erc-autojoin-channels-alist'.