The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Fixes potential crash.

Check for 0 in "device" and "inode" instead.

Remove the C list_head library and use type-safe C++ instead.

Fixes crash bug because mpd_connection_get_async() was called without
a connection.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Move to SimpleDatabase::GetSong() to give that method more control.

Code can now be reused in LookupSong().

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

The new method IsPlugin() replaces the "is_simple" flag.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Don't use the global variables from the DatabaseSimple library.

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.