libwrap is an obscure artefact from a past long ago, when source IP
address meant something.

And its API is "interesting"; it requires the application to expose
two global variables `allow_severity` and `deny_severity`.  This led
to bug #437.  I don't want to declare those variables; instead, I'd
like to remove libwrap support.

Closes #437

Prepare for per-listener permissions.

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

Closes #169

Passing it by value is actually smaller (32 bit) than the rvalue
reference (64 bit pointer), and it ensures that the object is consumed
after the call returns, no matter how the methods are implemented.

Prepare to make it mutable.

Prepare to convert "partition" to a mutable pointer.

No GLib memory allocation.

Hard-code the program name to "mpd" instead of using g_get_prgname().
The latter has become useless since this GLib variable doesn't get
initialized anymore.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

More preparations for multi-player support.

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

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