Don't call Seek() if the stream is already at the beginning.  This
avoids unnecessary exceptions if seeking is not implemented by an
Inputstream implementation.

This reverts commit ff3e2c05.  The
check was necessary, after all, because this is what checked whether
the decoder had finished the current or the next song.

> The "queued" flag can only possibly be set if the decoder is still
> decoding the current song or if the decoder is stopped.

That was wrong because ProcessCommand() sets `queued=true` and also
starts the decoder (if it was idle).

> This is also what the following assert() checks.

That was also wrong, because the assert() has two conditions.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/566

Closes https://github.com/MusicPlayerDaemon/MPD/issues/568

According to http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#sections

linux/limits.h defines MAX_INPUT which breaks the build.

Supports suffixes such as "kB" and "MB".

PluginUnconfigured exceptions are logged with level "info" instead of
"error".  This suppresses some rather boring messages in the default
log level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/565