Publish the Client API, preparing to move more code into the Client
class.

Automatic memory management.

Both callers pass non-nullptr.

Lifts the artificial 31 bit limit in i386 where
sizeof(long)==sizeof(int).

The "loop_count" configuration parameter allows the user to set how
many times a module with backward loops shall loop. "0" (the default)
means a module is not allowed to use backward loops at all. "-1"
enables inifinite looping.

Now config_param::GetBlockValue() can be used to get signed integers
from the configuration.

Fixes a regression from 752dfb3d which
caused the current chunk to be flushed as soon as new replaygain
information was found. If this occurs on a tag chunk, it has no data
(length 0) and is then skipped before pushing it to all of the outputs.

This change allows 0-length chunks through if they contain a tag and
they are now appearing in mplayer and mpv properly.