The old struct config_param remains only for top-level string options.

Simplify the definition of many build options.

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.

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.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Simpler to use than const_cast.

Allow database plugins to announce that they have been modified.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.