Another big chunk of code for multi-player support.

The pkg-config file has been added in Samba 4.0.  This commits adds a
fallback for older libsmbclient versions.

Allow semicolons and single quotes in the stream title.  This is not
part of any specification, but found in real life.

We're C++, and sparse understands only plain C.

Was broken on out-of-tree builds.

This was just too complex and fragile.  Anybody who has versioned
program names shall just call those programs manually.

I expect that aclocal is clever enough to find its own include
directories.  If it does not, letting MPD guess it is not the most
reliable idea either.

This call was unnecessary.  It is a bad idea anyway to block the MPD
process for some arbitrary amount of time.

Eliminate class LibUPnP and move the code to ClientInit.cxx.  Its
initialization function can be called multiple times, but
UpnpRegisterClient() is called at most once.

Allow calling UpnpGlobalInit() multiple times.

Each Upnp*Async() call passes a new cookie pointer, and the cookie
passed to UpnpRegisterClient() appears to be ignored.  Using this
interface is a more elegant approach than having one single "handler"
function.

This invalidated the std::string instance, and thus broke the CUE
parser (commit 322b0616).

Simplifies test program rules.

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.

This appears to be the right thing to do when we want anonymous login.