Prepare for option values.

Allow the caller to use a simple "for" loop without checking
arguments.

This implementation behaves mostly identical to old parser.
Few observable differences:
- There are no option groups (single group is used for all options)
- Option --stdout is hidden (it has been obsolete for a long time)
- MPD executable name (mpd) is hardcoded for simplicity

BufferedSocket has just an input buffer, and FullyBufferedSocket adds
the output buffer.

Don't use a global loop over the whole client list.

Use std::list instead of GList.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The soure file client.c has nearly 1000 lines, time for splitting it
into smaller pieces to improve readability.