Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Fixes hanging child process in the "pipe" output plugin.

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Eliminate the global variable "shutdown_loop".