Use the new HugeAllocator as backend for SliceBuffer and call
HugeDiscard() when the last chunk was returned.

Avoid page faults on MPD startup.  This saves a lot of memory for an
idle MPD.

This is harmful for memory usage, because it forces the kernel to
allocate physical memory, even before playback has started.  No bug
has been found in a few years with this, so it's safe to assume that
this code is not necessary.

Random access is not necessary here.

.. instead of rolling own classes.

Make valgrind happy.

Move code from client_new() and client_close().

Track whether _finish() has been called, and deliver all partial
results then.  Fixes Mantis ticket 0003621.

Rationale: vanilla libid3tag does not have any pkg-config stuff
and fails to detect because symbols from libz are not found.

- fix potential memory leak of system_path

  'Potential' because currently g_get_system_config_dirs()
  returns single entry on Windows, but that might change.

- remove incorrect g_free() call

  It's not required at all because
  g_get_system_config_dirs() returns GLib owned memory.

- remove extra semicolon