Add a Page queue to class HttpdOutput, and use DeferredMonitor to
flush this queue inside the IOThread.  This fixes a thread-safety
issue: much of EventLoop is not thread-safe, and the httpd plugin
ignored that problem.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Eliminate the C wrappers.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Remove trailing whitespace found by this command:
find -name '*.[ch]' | xargs grep "[[:space:]]$"

Avoid buffer underruns on the streaming client, if the encoder is "too
efficient" (e.g. when encoding silence while paused).

Implement the methods enable() and disable().  Bind the HTTP port in
the enable() method, but reject all incoming connections until the
output is opened.

[mk: folded with patch "Put icy related functions in extra source
files"; moved icy_server.c from HAVE_CURL to ENABLE_HTTPD_OUTPUT;
removed an unused variable]