Without the flush, ReadPage() may not return any data, or not all
data.  This may result in incomplete ddata the new "header" page,
corrupting streams with some encoders such as Vorbis.

Fixes #145

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

Fixes another buffer overflow: if the stream has a very long title or
URL, resulting in a metadata string of more than 2 kB, icy_string[0]
is a negative value, which gets casted to size_t - ouch!

 https://bugs.musicpd.org/view.php?id=4652

Fixes a buffer overflow due to the bad formula rounding the buffer
size up.  At the same time, remove the "+1" from the meta_length
calculation, which takes the padding into account and at the same time
implements proper rounding.

Why did this cast exist??

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

The old struct config_param remains only for top-level string options.

Necessary for "final" on gcc 4.6.