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.