Replaces StatFile(), with a portable data object.

PathExists() should better do what CheckAccess() does, and
CheckAccess() doesn't do what its name implies.

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

Return false on error, not nullptr.

New in clang 3.6.

Simplify the definition of many build options.

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

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

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.

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.