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This adds on-behalf-of="#id" to <listen-socket>. It allows a socket
to handle the traffic that was originally meant of another (virtual)
listen socket.

This moves all the listen socket code into a nice and abstracting file.

Notes:
* Altering listen socket setup does not yet work on config reload.
  (Did it ever work?)
* Server will start with no listen sockets. (There are unconfirmed
  rumours it sometimes(?) did before.) This is to be re-implemented
  in another commit. It can also be improved to work allow checking
  on reload or other config changes.
* For slave connections the server address is now checked against
  the allow/deny-IP list.

This allows ICY source clients to log in again with 2.4.x-style config.
ICY does not provide a username. Therefore the username was unset.
This patch sets the username for ICY source connections and allow them
to be handled as if a username was provided.
Configuration tag <shoutcast-user> was added to select the username to set
to. This defaults to "source".

This fixes a number of segfault happening in case config tags are
empty. This patch also adds warnings for most cases.

Please test for any regressions.
This should likely be ported to 2.4.4 as well.

Closes: #2265

As proposed in #2272, this renames the mount specific
mp3-metadata-interval to icy-metadata-interval to prevent
confusion about what it does (setting the icy metadata
interval) and to make clear it's not mp3 specific but ICY
specific.

This fixes the issue that auth for STATS method always fails, by
adding stats method to legacy admin auth in cfgfile.c