Auto-stop the IO thread in all error handlers.

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.

Convert filesystem charset to UTF-8 for playlist_list_open_uri().
This fixes one of many remaining charset bugs.

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

Combines the "file" and the "archive" input plugins.

This special case was useless.  Fixes the dreaded "Unrecognized URI"
error message when a file does not exist.

"override" implies "virtual".

Found by "make distcheck".

If an async opertion is in progress, nfs_close_async() will make
libnfs crash because the RPC callback will dereference an object that
was freed by nfs_close_async().

Close the newly allocated file handle passed to the callback.

All locks are currenly held from only a single thread (the IOThread)
and thus we don't need the Mutex.

Not used anymore because NfsManager now uses boost::intrusive::set
instead of std::map.

Reduce overhead for storing the key twice, and more overhead while
looking up the connection to remove it after a failure.

This allows eliminating the indirection code from the NfsConnection
destructor.

It's just a useless wrapper for class NfsManager.