The poll() function is broken in some darwin versions.

This allows easily enabling a sanitizer by passing --with-sanitizer=
when configuring.

Fix a double free that could happen when the cache was cleared
during config reload and then subsequent XSLT requests would try
to evict the same cache item, because the item name was not set to
NULL in clear_cache_entry.

Previously this was no problem as a cache entry would only be evicted
on load of a new XSLT and that one would immediately fill in the place
of the old one.

The xmlBuildURI function only deals with URIs, not paths, therefore
the admin path has to be converted to a URI first.

The XSLT loader caches the admin path from the config file and
invalidates it by comparing the stored path with the one in the config.
But to the stored path a / is appended, which means the cache would
always invalidate as it never matches.

Closes: xiph/icecast-server#2332

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.

The URL is constructed considering Protocol, TLS Mode, Host, and Port.

This considers:
 - The Host:-header as provided by the client,
 - The effective listensocket of the client,
 - Global configuration.

This works for:
 - Playlist generation,
 - Admin authed playlist generation,
 - Stats based XSLT.

It also unifies code:
 - Default stats values.

Not yet included:
 - YP requests.