Share the NFS connection with the NFS input plugin.

Read all directory entries into memory and close the struct nfsdir
before returning the StorageDirectoryReader instance.  This is what
libnfs does, anyway.

Creating a NfsStorage sets its own export_name as the "base".  Now
NfsFileReader can use this information to derive the export_name to be
mounted, instead of guessing.  This solves the "too many connection"
problem on the NFS server while updating the database.

"override" implies "virtual".

Assume the configured filesystem character set is also used by the NFS
server.

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.

This was accidently disabled when storage plugins were introduced.

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

This is the backend for the upcoming "mount" command.

Build the UTF-8 version of the path automatically in the constructor.

Replaces map_to_relative_path() from Mapper.cxx.

Hide inside CreateLocalStorage().

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.

Prepare for the plugin interface.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.