Return `404 not found` for some common well-known paths, as clients requesting them usually do that automatically and don't expect endless audio stram.

Closes #572

When `metadata_sent` is `false`, the plugin assumes there is metadata
which must be sent, even if no metadata page was passed to the plugin.
Initializing it to `true` avoids dereferencing this `nullptr`.

Fixes #412

This code was added in 21851c06 but
looks completely broken:

- the status code is "206 OK" but "206" would be "Partial Content"

- the "Content-Length" header has a bogus value

- the "Content-RangeX" parameter has different bogus values (why
  "Content-RangeX" anyway and not "Content-Range"?)

Apart from that, there are strange undocumented non-standard headers
which are probably there to work around bugs/expectations in one
broken proprietary client product.  But these days, MPD doesn't bend
over to support broken clients.  So let's kill this code.

Closes #304

Passing it by value is actually smaller (32 bit) than the rvalue
reference (64 bit pointer), and it ensures that the object is consumed
after the call returns, no matter how the methods are implemented.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Don't let the server care for client problems.

Don't iterate the std::list each time.

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

Replaces GLib's GError.